General Chemistry Discretes Test Time: 30 Minutes Number of Questions: 30 This test consists of 30 discrete questions—questions that are NOT based on a descriptive passage These discretes comprise 15 of the 77 questions on the Physical Sciences and Biological Sciences sections of the MCAT MCAT GENERAL CHEMISTRY DISCRETES TEST DIRECTIONS: The following questions are not based on a descriptive passage; you must select the best answer to these questions If you are unsure of the best answer, eliminate the choices that you know are incorrect, then select an answer from the choices that remain Indicate your selection by blackening the corresponding circle on your answer sheet A periodic table is provided below for your use with the questions PERIODIC TABLE OF THE ELEMENTS H 1.0 He 4.0 Li 6.9 Be 9.0 B 10.8 C 12.0 N 14.0 O 16.0 F 19.0 10 Ne 20.2 11 Na 23.0 12 Mg 24.3 13 Al 27.0 14 Si 28.1 15 P 31.0 16 S 32.1 17 Cl 35.5 18 Ar 39.9 19 K 39.1 20 Ca 40.1 21 Sc 45.0 22 Ti 47.9 23 V 50.9 24 Cr 52.0 25 Mn 54.9 26 Fe 55.8 27 Co 58.9 28 Ni 58.7 29 Cu 63.5 30 Zn 65.4 31 Ga 69.7 32 Ge 72.6 33 As 74.9 34 Se 79.0 35 Br 79.9 36 Kr 83.8 37 Rb 85.5 38 Sr 87.6 39 Y 88.9 40 Zr 91.2 41 Nb 92.9 42 Mo 95.9 43 Tc (98) 44 Ru 101.1 45 Rh 102.9 46 Pd 106.4 47 Ag 107.9 48 Cd 112.4 49 In 114.8 50 Sn 118.7 51 Sb 121.8 52 Te 127.6 53 I 126.9 54 Xe 131.3 55 Cs 132.9 56 Ba 137.3 57 La * 138.9 72 Hf 178.5 73 Ta 180.9 74 W 183.9 75 Re 186.2 76 Os 190.2 77 Ir 192.2 78 Pt 195.1 79 Au 197.0 80 Hg 200.6 81 Tl 204.4 82 Pb 207.2 83 Bi 209.0 84 Po (209) 85 At (210) 86 Rn (222) 87 Fr (223) 88 Ra 226.0 89 Ac † 227.0 104 Unq (261) 105 Unp (262) 106 Unh (263) 107 Uns (262) 108 Uno (265) 109 Une (267) * 58 Ce 140.1 59 Pr 140.9 60 Nd 144.2 61 Pm (145) 62 Sm 150.4 63 Eu 152.0 64 Gd 157.3 65 Tb 158.9 66 Dy 162.5 67 Ho 164.9 68 Er 167.3 69 Tm 168.9 70 Yb 173.0 71 Lu 175.0 † 90 Th 232.0 91 Pa (231) 92 U 238.0 93 Np (237) 94 Pu (244) 95 Am (243) 96 Cm (247) 97 Bk (247) 98 Cf (251) 99 Es (252) 100 Fm (257) 101 Md (258) 102 No (259) 103 Lr (260) GO ON TO THE NEXT PAGE as developed by General Chemistry Discretes Test Which of the following statements about molecular and empirical formulas is (are) FALSE? I A given compound can have the same molecular and empirical formula II The empirical formula is a whole number multiple of the molecular formula III H2O2 represents the empirical formula of hydrogen peroxide A B C D III only I and II only II and III only I, II, and III All of the following statements are consistent with Bohr’s model of the atom EXCEPT: A an electron may assume an infinite number of velocities B an electron is most stable in its ground state C the electron shell numbers represent the principal energy levels D electrons in orbitals closest to the nucleus have the lowest energy What volume of water would be needed to dilute 50mL of 3M H2SO4 to 0.75M? A B C D 50 mL 100 mL 150 mL 200 mL Among the elements with partially filled p subshells, atomic radii decrease significantly from left to right across a period while the atomic radii of transition elements change only slightly from left to right across a period This difference is most likely due to: A the presence of greater numbers of electrons in the outer shells of transition elements B the presence of greater numbers of electrons in the outer shells of elements with partially filled p subshells C the greater nuclear charges of transition elements D the presence of d electrons in the second outermost shell in transition elements H2O has a higher boiling point than HF because: Which of the following statements is NOT true of a gas at pressures greater than 400 atm and temperatures close to K? A The pressure of the gas is always lower than that predicted by the ideal gas law B The extent to which the gas deviates from the ideal gas law depends on the molecules’ size and polarity C Intermolecular forces become significant D The ideal gas equation is less accurate under these conditions than at higher temperatures and lower pressures A B C D H2O is more polar than HF H2O can form more hydrogen bonds H2O has a higher molecular weight H2O has more atoms Which of the following compounds contains the greatest percentage of oxygen by weight? A B C D C3H6O5Cl C3H6O2 C5H10O5 C4H8O3 GO ON TO THE NEXT PAGE KAPLAN MCAT Which of the following conditions guarantees a spontaneous reaction? A B C D Positive ∆H, positive ∆S Positive ∆H, negative ∆S Negative ∆H, negative ∆S Negative ∆H, positive ∆S Which of the following statements concerning equilibrium constants is true? A An equilibrium constant that is close to means that the reaction does not favor either reactants or products heavily B Equilibrium constants have units of L/mol C The stoichiometric coefficients in a reaction are not part of the equilibrium expression D The equilibrium constant for a reaction is the same for any temperature The Ksp of Mg(OH)2 in water is 1.2 × 10–11 mol3/L3 If the Mg 2+ concentration in an acid solution is 1.2 × 10–5, what is the pH at which Mg(OH)2 just begins to precipitate? A B C D 11 The rate law expression for the following reaction: N2 + H2 → NH3 A B C D can be represented by rate = [NH3]2 / [N2][H2]3 can be represented by rate = k[NH3]2 can be represented by rate = k[NH3]2 / [N2][H2]3 cannot be determined from the information given K+ and Cl– have the same: If the pressure of a gas sample is doubled at constant temperature, the volume will be: A B C D atomic weight electronic configuration ionization constant number of protons and neutrons A times the original B times the original Which of the following characterizes a galvanic cell? A B C D relates resistance and voltage relates voltage and current relates resistance and concentration relates concentration and voltage The melting points of ionic solids are: A B C D high because of their electrostatic attractions high because of their high densities low because of their intermolecular attractions low because of their high dissociation constants correctly I Oxidation occurs at the anode, which is negative II Oxidation occurs at the anode, which is positive III Reduction occurs at the cathode, which is positive of the original D of the original C 1 The Nernst equation: statements A B C D II only III only I and III only I, II, and III In order to make a buffer solution, a weak monoprotic acid could be added to: A B C D another acid another base the salt of its conjugate base the salt of its conjugate acid GO ON TO THE NEXT PAGE as developed by General Chemistry Discretes Test When chromium metal is used to form K2Cr2O7 the oxidation state of chromium changes from: A B C D to to to to 6 Which of the following molecules contains both ionic and covalent bonds? A B C D C6H14 MgCl2 (NH4)2SO4 H2O What is the range of possible values for the [OH–] / [H+] ratio in an aqueous acid solution? A B C D 0–1 – 14 – 14 1–∞ A flask contains three times as many moles of H2 gas as it does O2 gas If hydrogen and oxygen are the only gases present, what is the total pressure in the flask if the partial pressure due to oxygen is Pl? A 4P B 3P P 3 D P C Which of the following will increase the rate at which ice melts in a closed container if all other parameters are kept constant? A B C D Adding water with a temperature of 0°C Lowering the temperature below 0° C Lowering the pressure Raising the pressure Which of the following molecules is polarized? A B C D BH3 NF3 C2H6 SF BaCl2 dissociates in water to give one Ba2+ ion and two Cl – ions If concentrated HCl is added to this solution: A B C D [Ba2+] increases [Ba2+] remains constant [OH–] increases the number of moles of undissociated BaCl2 increases 2 Avogadro’s number is NOT equal to: A B C D Which of the following is NOT a true statement about the entropy of a system? A Entropy is a measure of the randomness in a system B The entropy of an amorphous solid is greater than that of a crystalline solid C The entropy of a spontaneous reaction cannot decrease D At constant temperature and pressure, the entropy of a system will spontaneously increase the number of atoms in 11.2 L of O2 at STP the number of atoms in mole of He at STP the number of electrons in 96,500 coulombs the number of SO GO ON TO THE NEXT PAGE ions in L of N sulfuric acid KAPLAN MCAT From the following reaction and thermodynamic data, what is the energy released when a C—H bond forms? (Note: ∆Hf° CO2(g) = –393 kJ/mol, and ∆Hf° H2O(g) = –242 kJ/mol.) CH4 + O2(g) → CO2(g) + H2O(g) ∆H°rxn = – 802 kJ/mol A B C D –75 –19 75 167 kJ/mol kJ/mol kJ/mol kJ/mol What would be the stoichiometric coefficient of hydrochloric acid in the following equation? Cl2 + H2O → HCl + HClO3 A B C D 10 Which of the following compounds can act as a Lewis base? Which of the following generalizations CANNOT be made about the phase change of a pure substance from solid to liquid? A B C D It involves a change in potential energy It involves no change in temperature It involves a change in kinetic energy It involves a change in entropy A HClO2 B NH2NH2 C NH4+ D BF3 END OF TEST as developed by General Chemistry Discretes Test THE ANSWER KEY IS ON THE NEXT PAGE KAPLAN MCAT ANSWER KEY: C 11 D A 12 A A 13 D C 14 D D 15 B 10 B C D A C 16 17 18 19 20 C C D C D 21 22 23 24 25 D D A A B 26 27 28 29 30 C B C C B as developed by General Chemistry Discretes Test GENERAL CHEMISTRY DISCRETES TEST EXPLANATIONS The answer to question is choice C To answer this, you must understand the differences between empirical and molecular formulas The empirical formula of a compound is the simplest whole number ratio of the atoms in it, whereas the molecular formula represents the actual whole number ratio of the atoms The molecular formula is usually a whole number multiple of the empirical formula, although sometimes the empirical and molecular formulas are the same Given this information, let's go through the statements to determine which of them is false Statement number I is true For instance, water, H2O, has the same empirical and molecular formula Statement II is false because it is the molecular formula which is usually a whole number multiple of the empirical formula, not the other way around Finally, statement III is false H2O2, hydrogen peroxide, has an empirical formula of HO, so the empirical formula is different from the molecular formula given Another is glucose, C 6H 12O 6, which has an empirical formula of CH2O Since statements II and III are both incorrect, choice C is the answer we're looking for In question 2, the right answer is choice A Let's go through each of the choices and see how they apply to Bohr's model of the atom Choice A, which says that an electron may assume an infinite number of different velocities, is true in classical mechanics but not in Bohr's model Bohr used quantum theory in developing his atomic model and placed specific conditions on the possible values of the electron velocity Since we're looking for the incorrect statement, A is the correct answer Choice B, which says that an electron is most stable in its ground state, is true The ground state of an electron is its lowest possible energy state From thermodynamics, we know that systems tend to be the most stable at low energy; thus, an electron is most stable in the ground state Choice C says the electron shell numbers represent the principal energy levels This is true; the principal quantum number of a given electron is the same as the number of its electron shell in the Bohr model Choice D says that electrons closest to the nucleus have the lowest energy As an electron absorbs energy, it jumps to a higher orbital and increases its distance from the nucleus The electron drops to a lower orbital releasing energy or it escapes the pull of the nucleus, making the atom a cation Again, choice A is the correct answer For question 3, the correct choice is A One is asked in this question to determine which of the choices contains a false description of the gas Under conditions of high pressure and low temperature, the gas is not behaving ideally and corrections must be made for the volume of the gas molecules and the intermolecular forces between them Therefore, choice D, which states that the ideal gas equation would not describe the gas accurately is quite correct Choice C is also a characteristic of the gas since the high pressure reduces the volume and thus the distance between the molecules Intermolecular forces, therefore, have a greater effect on each gas molecule Now we have narrowed our choices down to A and B The van der Waals equation is useful in describing and predicting the behavior of non-ideal gases, and one of its features is the introduction of two parameters a and b, which account for intermolecular forces and excluded volumes respectively The values of these parameters depend on the gas in question; more specifically, the magnitude of a is determined by how strongly the gas molecules attract one another, and b is influenced by the size of the molecules The higher the values of a nd b, the more the behavior of the gas is non-ideal (Recall that if a =b=0, one gets back the ideal gas law.) Therefore choice B is also correct By elimination, one can hence conclude that the correct answer is choice A because it contains a false statement In rationalizing the van der Waals equation, one observes that the presence of intermolecular attraction leads to a lower pressure than would be expected from using the ideal gas law, but when coupled with the effect of excluded volume, no generalization can be made as to whether the actual or ideal pressure would be greater (This holds for gas volume as well, unless if condensation has taken place, in which case the volume occupied by the liquid will be much smaller than that predicted by any gas law.) For question 4, which is a simple dilution problem, the correct answer is C The number of moles of solute in the solution will be the same after dilution as before, and the number of moles in each case is equal to the molar concentration multiplied by the volume of solution This means that the initial concentration times the initial volume will be equal to the final concentration times the final volume So, the final volume will equal the initial concentration times the initial volume divided by the final concentration If we work out the math, we find that the final volume of the diluted solution will be 0.2 liters, or 200 milliliters Since we started out with 50 milliliters of the solution, we would have to add 150 milliliters of water to get a final volume of 200 milliliters Therefore, the correct answer is choice C Question is correctly answered by choice D To answer this question, we have to think about the thing that determines atomic radius: how strongly the outermost electron shell is attracted to the nucleus Electrons are attracted to the nucleus by its positive charge There are two factors that affect the strength of that attraction: more protons for a stronger pull by the nucleus, which reduces the atomic radius, and the shielding of the outermost electron shell by the intervening filled electron shells, which increases the atomic radius So trends in atomic radii result from interaction between these two competing effects The key thing here is that the electrons in the outer shell are not very effective at shielding each other from the nuclear pull, while electron shells further in toward the nucleus have a strong shielding effect on electrons in the outer shell Now, because of the order in which electron orbitals fill, as you go from left to right across the table, each subsequent transition element generally has one more d electron but doesn't change the number of electrons in the outermost s subshell However, when you look at the elements with the partially filled p subshells, each subsequent element has one more p electron The d electrons are in the next-to-last shell, so they shield the valence s shell pretty effectively Thus, even though the number of protons in the nucleus increases from left to right across the table, the fact that the number of d electrons is increasing at the same time means that the atomic radii don't decrease significantly Therefore, choice D is the correct answer By contrast, the elements with the partially filled p subshells are adding electrons to their p subshell, which is their outermost subshell Thus, choice B is wrong because it does not explain the strange trends in transition metal radii KAPLAN MCAT Choices A and C are wrong because within a given period, the transition elements have fewer electrons and smaller nuclear charges that the elements with partially filled p orbitals Once again, choice D is the right answer The correct answer to question is choice B This question asks you to determine why water has a higher boiling point than hydrogen fluoride Choice A says that water has a higher boiling point because it is more polar Before you decide whether polarity has an effect on the boiling point of a substance, you should be able to eliminate this choice by realizing that water is not more polar that HF Hydrogen fluoride has a greater charge separation between its constituent atoms and thus has a more polar bond Choice B says that water has a higher boiling point because it can form more hydrogen bonds Hydrogen bonding does affect boiling points by increasing the attraction between the molecules of a compound Water is capable of forming as many as four hydrogen bonds per molecule while hydrogen fluoride can only form This intermolecular attraction leads to a complexation of water molecules and contributes to the high boiling point of water Thus choice B is correct Although we have found the right answer, let's look at the rest of the choices Choice C says that water will have a higher boiling point because it has a higher molecular weight Well, this statement is also false in and of itself because hydrogen fluoride, at 20 grams per mole, has a greater molecular weight than water, which is only 18 grams per mole Finally, choice D says that water has more atoms than hydrogen fluoride Well, this is true, but it has nothing to with boiling point, so is not the answer we're looking for So, the correct answer is choice B For question 7, the correct answer is C You need to first calculate the weight of oxygen in each compound and then divide that value by the compound's molecular weight The weight of oxygen in choice A is 80 grams per mole and the molecular weight is 157 grams per mole Dividing this out gives 51% oxygen For B, the weight of oxygen is 32 grams per mole and the molecular weight is 74 grams per mole This gives you 43% oxygen Choice C contains 80 grams of oxygen per mole and its molecular weight is 150 grams per mole Here, you get 53% oxygen Finally, choice D contains 48 grams of oxygen per mole and its molecular weight is 104 grams per mole, which yields 46% oxygen Obviously, choice C contains the most oxygen by weight and is the correct answer The correct answer for question is D This question asks you to predict which combination of ∆H and T∆S values will always give a spontaneous reaction Recall that spontaneous reactions have negative values of Gibbs free energy, ∆G, and that ∆G equals ∆H – T∆S, where ∆H is the change in enthalpy, ∆ S is the change in entropy, and T is the absolute temperature From this equation, it's clear that the best way to guarantee a negative ∆G is to have a negative ∆H value and a positive ∆S value, since T, always expressed in Kelvin, is always positive This corresponds to choice D, so that is the right answer For question number 9, choice A is correct To answer this question, you need to understand the concept of equilibrium and know the expression for the equilibrium constant Let's go through all of the choices Choice A says that if an equilibrium constant is close to 1, then neither reactants nor products are favored heavily This is correct because the equilibrium constant expresses a ratio of product concentrations to reactant concentrations If the reaction is heavily in favor of products, the value of the equilibium constant will be much greater than Conversely, if the reaction favors reactants, then the equilibrium constant would have a value much less than one Choice B says that equilibrium constants have units of L/mol This is incorrect; the equilibrium constant is actually a dimensionless number Choice C states that stoichiometric coefficients from the reaction have nothing to with the equilibrium expression The equilibrium expression is derived by multiplying the concentrations of the products raised to their stoichiometric coefficients together and then dividing by the product of the reactant concentrations raised to their stoichiometric coefficients So we know choice C is wrong Finally, choice D says that the equilibrium constant of a reaction is the same for any given temperature This is wrong because an equilibrium constant is a characteristic of a system at a particular temperature Changing temperatures changes the system identity, so it also changes the equilibrium constant Again, the correct answer is choice A 10 For question 10, the right answer is choice C The question is an application of Boyle's Law This states that at constant temperature, the pressure and volume of a gas are inversely proportional to each other Therefore, since the pressure of the gas in the question increased, the volume must decrease Since the pressure and volume are inversely related, when the pressure is doubled, the volume is halved This is choice C 11 The answer to question 11 is choice D The Nernst equation expresses the relationship between the observed potential of a redox system and the concentrations of the reactants and the products involved It is expressed as follows: the potential of the cell, E, is equal to the standard potential of the cell minus the quantity RT over nF times the natural log of the reaction quotient, Q R is the universal gas constant expressed in joules per mole Kelvin, T is the absolute temperature, n is the number of moles of electrons exchanged in the redox reaction, and F is Faraday's constant Q is just a representation of the extent to which a reaction has proceeded Since potential is expressed in volts and Q is expressed in concentration units, the equation relates voltage and concentration and the correct answer is D 12 Moving on to question 12, the correct answer is A Because an electron is transferred from one atom to another in an ionic bond rather than being shared as in a covalent bond, the atoms in an ionic solid exist as ions and are held together by very strong electrostatic forces Because these forces hold them in tight crystalline patterns, ionic solids generally have high melting points This makes choice A correct Density, as in choice B, doesn't explain the high melting points of ionic solids since substances can be very dense and still melt easily The concept of intermolecular forces, choice C, not really apply in the case of ionic solids because there are no discrete molecules within such a solid: it is a giant lattice of positive and negative ions Further, choice D is also wrong because even though ionic solids tend to dissociate easily, this 10 as developed by General Chemistry Discretes Test has nothing to with melting points For instance, table salt, NaCl, will readily dissociate and dissolve in water, but if you were to simply place the solid on a hot plate, it would take a long time to melt Therefore, choice A is the best answer 13 Question 13 is appropriately answered by choice D This question deals with solubility constants and the common ion effect The first step to solving this problem is to express the solubility product constant, which is the ion product of the saturated solution, as the product of the concentration of Mg2+ ion and the concentration of the hydroxide ion squared Second, you must determine the minimum concentration of hydroxide necessary to precipitate the Mg(OH)2 From the equation just given, you can solve for the concentration of hydroxide, which is the square root of the K sp divided by the concentration of magnesium Substituting in the values provided in the question, you should find that the concentration is equal to ten to the minus moles per liter That means that the pOH of the solution, the negative log of the hydroxide ion concentration, will be Since the pOH plus the pH of any solution must equal 14, the pH must be 11 This corresponds to choice D 14 The correct answer to 14 is D This question asks you to determine the rate law of the reaction for the formation of ammonia, NH3 A rate law is an equation which gives the relationship between the rate of the reaction and the concentration of the reactants, each raised to an appropriate power which will depend on the exact reaction For example, the rate of this reaction is equal to k times [N2] raised to the x power times [H2] raised to the y power, where k is the rate constant and x and y are real numbers x is called the partial order of the reaction with respect to N and y is the partial order of reaction with respect to H2 x plus y gives the overall order of reaction k, x, and y can only be determined experimentally by systematically varying the initial concentration of one reactant In particular, there is no relationship between the coefficients of the balanced equation and the order of reaction In this question, choices A, B, and C show rate laws which contain the concentrations of the product, ammonia, and the actual values for the partial orders of reaction However, since there is no experimental data, there is no way for us to know what these values are, so these three choices are wrong The only correct answer is D, which says that the rate law cannot be determined from the information given 15 The right answer for question 15 is B The atomic weight of an element depends on the number of protons and neutrons in its nucleus; those numbers are always unique to a particular element, regardless of the number of electrons in the species Therefore, choices A and D are incorrect The ionization constant, choice C, depends chiefly on the radius of the parent atom and the effective charge of the nucleus Since K+ and Cl – have different numbers of protons by definition, they will have different effective charges, different atomic radii, and therefore different ionization constants, so choice C is wrong Choice B, however, says that the two ions have the same electronic configuration Well, chlorine belongs to the third period and potassium belongs to the fourth, so in their unionized forms, chlorine's third shell contains seven electrons and potassium's fourth shell contains one electron If chlorine gains one electron and potassium loses one electron, both will have eight electrons in the third shell, which becomes the valence shell So the potassium and chlorine ions described in the question both contain the same number of electrons and the same number of occupied orbitals and thus share the same electronic configuration Again, choice B is the correct answer 16 For question 16, the correct answer is choice C This question asks which of the given statements describes a galvanic cell This is a roman numeral question, so remember that one or more of the answers may be correct The first thing to remember is that galvanic cells are capable of spontaneous reaction Let's go through each of the choices to see which of them are correct Statement I says that oxidation occurs at the anode, which is negative In all electrochemical cells, oxidation occurs at the anode and reduction occurs at the cathode In addition, the anode in a galvanic cell is negative, meaning that it is a source of electrons Since a species loses electrons when it is oxidized, this should make sense There is a trick to remembering these facts In a galvanic cell, oxidation occurs at the anode, which is negative Alphabetically, anode comes before cathode, oxidation before reduction, and negative before positive However, this little trick only works for galvanic cells, but that's what we've got here Thus, statement I is correct and statement II is wrong That eliminates choices A, B, and D, so the answer must be C Statement II says that the anode is positive in a galvanic cell We just talked about that being wrong, but you may have gotten confused since electrolytic cells have positive anodes Statement III is the compliment of statement I, so it must also be true That makes choice C the right answer 17 The correct answer to number 17 is choice C Buffers are solutions made from a mixture of a weak acid and a salt containing its anion They resist pH change due to the addition of acid or base Thus, to make a buffer solution with a weak monoprotic acid, the addition of the corresponding salt is required This salt must contain the anion, or conjugate base, of the acid Therefore, the way to make a buffer solution when you start with a weak monoprotic acid is to add the salt of the conjugate base to the solution This is choice C, which is thus the right answer If you add a strong acid to a weak acid, you just get an acidic solution, so choice A is wrong If you add a weak acid to a strong base, the base will neutralize the acid, so choice B is wrong Finally, the only way that an acidic species can have a conjugate acid is if it is polyprotic For example, the species HSO4– is acidic, but is the conjugate base of the acid H2SO4 We know that our acid is monoprotic, so choice D is way off the mark Again, the correct answer is choice C 18 Question 18 is choice D This question asks you to determine the change in the oxidation number of chromium when it goes from its elemental state into K 2Cr2O By definition, the oxidation number of any element in its elemental form is zero Choices B and C not have zero in them, so they can easily be eliminated Next, you should calculate the oxidation number of chromium in K 2Cr2O Since K 2Cr2O is a neutral molecule, the sum of the oxidation numbers of all of the atoms in it must be zero K is in group one A, so it has an oxidation number of +1 In most compounds, oxygen has an oxidation number of –2 Generally, any time you have to figure out the oxidation state of oxygen in a compound, it is safe to assume that it is –2 unless you have reason to suppose otherwise Okay, if you set up the KAPLAN 11 MCAT calculation, you get (2 × +1) + (7 × –2) for a total of –12 That means that between them, the two chromium atoms have to add up to +12 to balance out the molecular charge That means that in K2Cr2O7, each chromium atom has an oxidation state of +6, so chromium has gone from to +6, which is choice D 19 The correct answer for question 19 is choice C This question asks you which of the molecules given as answer choices contains both ionic and covalent bonds Choice A is hexane, which is a hydrocarbon This is wrong because all the bonds in hexane are covalent carbon-carbon or carbon-hydrogen bonds Choice B is magnesium chloride, which is a salt This only has ionic bonds, so choice B is wrong Choice C is ammonium sulfate The bonds between nitrogen and hydrogen in the ammonium ion and between sulfur and oxygen in the sulfate ion are all covalent However, the bond between the ammonium ion and the sulfate ion is an ionic bond Thus, ammonium sulfate has both ionic and covalent bonds, so choice C is correct Finally, choice D is water The oxygen-hydrogen bonds in water are covalent, so choice D is incorrect Again, choice C is the right answer 20 For question 20, answer choice D is correct This question asks you which of the choices will increase the rate at which ice melts in a closed container Okay, the first two choices both deal with temperature and of course it's intuitively obvious that temperature changes can alter the melting rate of ice Now, choice A says that the melting rate will be increased if water at zero degrees Celsius is added to the container But zero degrees Celsius is the temperature at which the solid and liquid forms of water are in equilibrium, that is, the melting or freezing point of water, so adding water at that temperature won't change the melting rate at all Choice B says that the rate will increase if the temperature is dropped below zero degrees Celsius This is even worse As I just said, zero degrees Celsius is the point at which the solid and liquid are in equilibrium, so if you lower the temperature, you are tipping the equilibrium toward the ice The last two choices are less intuitive They deal with pressure, and to evaluate them, we have to apply Le Chatelier’s Principle, which says that a system in equilibrium that is subject to stress will shift its equilibrium so as to relieve the stress If the pressure is lowered, as in choice C, the system will counteract the change in pressure by shifting its equilibrium toward the phase that is less dense In the case of water, that is the ice Remember that water has a strange property in that the solid form, ice, at zero degrees Celsius is less dense than the liquid phase, water, at that temperature Since the reduction of pressure drives the system to produce ice, this is the wrong answer However, choice D, an increase in pressure, will have the opposite effect and the water will be produced preferentially That means that the ice is melting faster and D is the correct answer 21 Choice D is the right answer for question To answer this question, you should know that when hydrochloric acid, a very strong acid, is added to a solution, it will dissociate completely into hydrogen ions and chloride ions This increases the concentration of chloride ions already in the solution from the barium chloride According to Le Chatelier's Principle, if the concentration of one reaction species is increased, the reaction will be driven in the opposite direction For this example and the dissociation reaction of barium chloride, if you increase the concentration of chloride ion, you will drive the reaction in the direction of reassociation of barium chloride Therefore, barium chloride will precipitate out of the solution That means that choice A, which says that the concentration of barium ion increases, and choice B, which says that the concentration of barium ion stays constant, are wrong Choice C says that the concentration of hydroxide ion will increase This is wrong because it will actually decrease so as to neutralize the hydrogen ions added by the hydrochloric acid Choice D says that the number of moles of undissociated barium chloride increases This is the same as saying that barium chloride precipitates out of solution, so choice D is the correct answer 22 The correct answer for question 22 is choice D Avogadro's number, 6.02 times ten to the twenty-third, is the number of molecules in one gram molecular weight of a substance, or one mole of the substance So the correct answer choice is the choice that does not describe one mole of a species You probably remember that a mole of gas at STP occupies a volume of 22.4 liters Choice A gives the volume of a half a mole of oxygen molecules, but since each molecule of oxygen gas has two oxygen atoms, choice A represents Avogadro's number of atoms Choice B, one mole of helium, is obviously a mole Choice C is the definition of a Faraday, which is one mole of electrons Choice D, the number of sulfate ions in a normal sulfuric acid solution is not a mole The reason is that there are two equivalents of hydrogen in each mole of sulfuric acid A one normal solution contains one mole of hydrogen ions, but only half a mole of sulfate ions One mole of sulfate ions from sulfuric acid would have to be normal So the right answer is choice D 23 The correct answer to question 23 is choice A The question asks you to give the range of possible values for the ratio of hydroxide ion concentration to hydrogen ion concentration in an aqueous acid solution Well, for a solution to be acidic, it must have a pH between about and 6.999 That means that the pOH of these solutions will be between 13 and 7.001 We know that pH is equal to the negative log of the hydrogen ion concentration in solution and the pOH is equal to the negative log of the hydroxide ion concentration in solution From this we can determine that the range of possible hydrogen ion concentrations in an acidic solution will be about 10 –1 to 10 –6.999 The corresponding range of concentrations of hydroxide ions in the same solutions will be 10–13 to 10–7.001 Now let's calculate the range of the ratio of hydroxide ions to hydrogen ions For the most acidic solution, the ratio will be 10 –13 divided by 10 –1 , which is equal to 10–12, or approximately For a slightly acidic solution, one very close to neutrality, the ratio will be 10 –7.001 divided by 10–6.999, which is nearly equal to one Thus, the range of the hydroxide ions to the hydrogen ions in an acidic solution is to 1, which corresponds to choice A From a purely mathematic point of view, it's even easier to see that in an acidic solution, the concentration of H+ will always be greater than the OH– concentration If the denominator of a fraction is always greater than the numerator, but both are positive numbers, then the limits will be and Again, choice A is the correct answer 12 as developed by General Chemistry Discretes Test 24 The answer for question 24 is choice A You are asked to determine the total pressure in the flask in terms of the partial pressure of the oxygen gas To this, you need to use Dalton's Law of partial pressures This law says that the sum of the partial pressures of all the gases in a given vessel is equal to the total pressure Since the partial pressure of oxygen is P and you know that there are three times as many moles of hydrogen as oxygen in the flask, then the partial pressure of hydrogen must be 3P1 Thus, the total pressure in the flask is P1 + 3P1, or 4P1, which corresponds to choice A 25 The answer to question 25 is choice B To answer questions about a molecule's polarity, you must think about the geometry of the molecule as well as the polarity of the individual bonds Of the four molecules given in the choices, three are symmetrical and so, even if their bonds are polar, the molecules themselves will not have a dipole moment According to the Valence Shell Electron Pair Repulsion theory, the three N-F bonds in NF point toward the vertices of a tetrahedron, and the nitrogen's lone pair of electrons points toward the fourth vertex, so the molecule has a trigonal pyramidal conformation The NF3 molecule is thus asymmetrical and will have a net dipole moment The electronegativity difference between nitrogen and fluorine shows that fluorine exerts a greater pull on the electrons in the N-F bonds, so the dipole moment will put a partial negative charge on the fluorine end of the molecule and a partial positive charge on the nitrogen, despite its lone pair of electrons Again, the correct answer is choice B 26 For question 26, the correct answer is choice C This question gives four statements concerning the entropy of a system and asks you to determine which one is incorrect Let's examine each one of the choices Choice A says that entropy is a measure of the randomness of a system This is the definition of entropy and is thus correct Choice B says that the entropy of an amorphous solid is greater than that of a crystalline solid This is true because an amorphous solid is less ordered than a crystalline solid and thus has greater entropy Choice C says that a spontaneous reaction will never have a decrease in entropy This is a false statement Depending on the temperature and the enthalpy change of a reaction, the entropy change can be negative For instance, in phase changes from gas to liquid or liquid to solid, a decreasing temperature spontaneously drives the system into the more ordered state That would be a decrease in entropy, so choice C is the answer to the question Although we found the answer, let's look at choice D to see how it applies to entropy Choice D says that at constant temperature and pressure, a system will spontaneously lose its order This is a basic premise of entropy because the second law of thermodynamics states that the entropy of the universe will increase Hence, choice D is a correct statement concerning entropy Again, the answer to this question is C 27 Looking at question 27, we get B as the right answer This question presents you with the combustion reaction for methane and asks you to calculate the energy of a carbon-hydrogen bond To begin this problem, you must use Hess' Law, which says that the heat of reaction is equal to the sum of the heats of formation of the products minus the sum of the heats of formation for the reactants The heat of reaction therefore equals the heat of formation of CO plus twice the heat of formation of H2O minus the heat of formation of CH4 minus twice the heat of formation of O2 The heat of reaction is –802 kJ/mol, the heats of formation of CO 2, H 2O, and O are –393 kJ/mol, –242 kJ/mol, and kJ/mol respectively Now you want to solve for the heat of formation for CH4 Since the heat of formation of a compound in its elemental state is always kJ/mol, the factor in our equation that allows for the heat of formation of O2 drops out Given this information, the equation to be solved is –802 kJ/mol = –393 kJ/mol plus times –242 kJ/mol minus x, where x is the heat of formation of methane Solving for x, we get –75 kJ/mol for the methane heat of formation Since there are four equal carbon-hydrogen bonds in methane, the energy of one bond is –75 kJ/mol divided by 4, or about –19 kJ/mol This is choice B 28 The correct answer to question 28 is choice C This question asks you which generalization CANNOT be made about the phase change of a pure substance from solid to liquid Choice A says that the phase change involves a change in potential energy When a phase change occurs, the internal energy of the system, that is, the total energy contained in the system, will change The potential energy of the system during the phase change is the same as the internal energy Therefore, when a solid melts into a liquid, the potential energy of the substance will change Choice B says that the phase change will occur at a constant temperature Solids have a defined temperature, known as a melting point, at which they change to liquid At this temperature, any energy added to the solid will go toward changing the phase, not changing the temperature, until all the solid has changed to liquid Thus choice B is true Choice C says that the phase change involves a change in kinetic energy We have already shown that the phase change occurs at a constant temperature, and because a change in kinetic energy is associated with a temperature change, the kinetic energy the solid to liquid transition will remain the same Hence, choice C is the answer to this question Choice D says that the solid to liquid phase change will involve a change in entropy This is true because molecules in the liquid state have more vibrational freedom and will therefore exhibit a greater degree of disorder Once again, the correct choice is C 29 In question 29, the answer is C To answer this question, you must balance the equation that's given to find the stoichiometric coefficient for hydrochloric acid Let's start with oxygen since it's the only element that is present in only one compound on each side of the equation Since there are three oxygens on the right side of the equation, you have to place a before the water molecule on the left There are now six hydrogen atoms on the left side of the equation, so a must be placed before the HCl on the right side for a total of six hydrogens on that side also Before jumping right to the answer, we should balance the entire equation, since the answer might be five or some multiple of five, depending on the other compounds There are six chlorines on the right, so you must place a three before the Cl on the left The equation is now fully balanced and it's clear that the answer is indeed 5, which is choice C 30 The correct answer to question 30 is choice B A Lewis base is a compound that can donate an electron pair Choice A, chlorous acid, will not act as a Lewis base Rather, it is an Arrhenius acid, capable of donating a hydrogen ion in solution Choice B, hydrazine, consists of two nitrogens bonded to each other and to two hydrogen atoms each A KAPLAN 13 MCAT nitrogen atom has a valence of five, and when it's bonded to another nitrogen and two hydrogens, it's only using three of its five valence electrons Therefore, each nitrogen will have a pair of unbonded electrons This makes hydrazine able to act as an electron-pair donor, or Lewis base, so choice B is the correct answer Looking at the other choices, choice C, an ammonium ion, is not a Lewis base In fact, it's the conjugate acid of ammonia Choice D is boron trifluoride The boron atom has three valence electrons, each of which is involved in a covalent bond with a fluorine atom, and thus has no electron pairs to donate Once again, choice B is the correct answer 14 as developed by ... 21 22 23 24 25 D D A A B 26 27 28 29 30 C B C C B as developed by General Chemistry Discretes Test GENERAL CHEMISTRY DISCRETES TEST EXPLANATIONS The answer to question is choice C To answer... (257) 101 Md (258) 102 No (259) 103 Lr (260) GO ON TO THE NEXT PAGE as developed by General Chemistry Discretes Test Which of the following statements about molecular and empirical formulas is... conjugate base the salt of its conjugate acid GO ON TO THE NEXT PAGE as developed by General Chemistry Discretes Test When chromium metal is used to form K2Cr2O7 the oxidation state of chromium