Chemistry Discrete Test 2

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General Chemistry Discretes Test 2

KAPLAN 1

General Chemistry Discretes

Test 2

Time: 30 MinutesNumber of Questions: 30

This test consists of 30 discrete questionsquestionsthat are NOT based on a descriptive passage. Thesediscretes comprise 15 of the 77 questions on thePhysical Sciences and Biological Sciences sections ofthe MCAT.


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KAPLAN 3

1. What is the formal charge on carbon in thecarbonate ion?

A. 0

B. -2

C. +2

D. +4

2. Which of the following represents the range ofvalues possible for the Azimuthal quantumnumber for an electron with principal quantumnumber n?

A. 0 to n-1

B. 0 to n

C. n to n

D. (n-1) to n-1

3. Which of the following is isoelectronic withAl3+?

A. Si+

B. B3+

C. O2-

D. Cl-

4.

temperature

pressure

*

In the diagram above, the starred point is thepoint at which:

A. all three phases are in equilibrium.

B. a substance changes from liquid to gas.

C. the differences between the properties of

the solid and liquid phases disappear.D. the differences between the properties of

the liquid and gas phases disappear.

5. Which of the following is not a Lewis base?

A. Cl-

B. NH3C. BF3D. THF (tetrahydrofuran)

6. For the equilibrium:

4 2 4 3HClO ( ) H O( ) ClO ( ) H O ( ),aq l aq l

+

+ +U

ClO4- is the:

A. conjugate acid of HClO4.

B. conjugate base of HClO4.

C. conjugate acid of H2O.

D. conjugate base of H2O.

7. Ten electrons have just been placed into 4dorbitals. The next electron will be placed in a:

A. 4forbital.

B. 4p orbital.

C. 5p orbital.

D. 5forbital.

8. Which of the following will occur if thebasicity of a saturated solution of Na2SO4 isincreased?

A. The Ksp will decrease and additional

Na2SO4 will dissolve.

B. The Ksp will be unchanged and additionalNa2SO4 will dissolve.

C. The Ksp will increase and additional

Na2SO4 will precipitate.

D. The Ksp will remain unchanged and

additional Na2SO4 will precipitate.

9. Which of the following is NOT true in generalas we move from sodium to chlorine in the

periodic table?

A. Electronegativity increases.

B. Atomic radius increases.

C. Ionization energy increases.D. Electron affinity increases.


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MCAT

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10. Ca(s) + F2(g) CaF2(s)

The reaction above is an example of all of thefollowing reaction types EXCEPT:

A. precipitation.

B. combination.

C. oxidation-reduction.

D. formation.

11. For the reaction below, what would be the effectof decreasing the volume of the system?

3 2 3 2 22NaHSO ( ) Na SO ( ) SO ( ) H O( )s s g+ +U g

A. The formation of products would be

favored.

B. The temperature of the system is required

to determine the effect.

C. The formation of reactants would befavored.

D. No change would occur.

12. Two identical containers each contain samplesof ideal gas at the same temperature. One (gas1) has a mass of 60 g/mol. The other (gas 2)has a mass of 30 g/mol. The relationship

between their average speeds is:

A.1 2

1

4v v=

B.1

1 22v v=

C.1 2

1

2v v=

D.1 2

3v v=

13. Below is a phase diagram for substance R.Which of the following conclusions can bedrawn about substance R?

temperature

pressure

solid

liquid

gas

A. The liquid phase of substance R contains

strong intermolecular bonds.

B. Substance R has a molar mass greater

than 30 g/mol.

C. The density of the gas phase is greater

than that of the liquid phase for substanceR.

D. The vaporization process for substance R

is exothermic.

14. A closed container holds four gases: 1 mole ofGas A, 1 mole of Gas B, 3 moles of Gas C and4 moles of Gas D. If the total pressure insidethe container is 3 atm, what is the partial

pressure of Gas C?

A. 0.33 atm

B. 1 atm

C. 1.33 atm

D. 9 atm

15. Two moles of a substance contains how manyparticles?

A. 6.022 1023

B. 12.044 1023

C. 6.022 1046

D. 12.044 1046

16. For the following skeletal reaction, what is thecoefficient for sulfur dioxide?

2 2 2 3 4 2CuFeS ( ) O ( ) Cu S( ) Fe O ( ) SO ( )s g s s+ + + g

A. 3

B. 5

C. 6

D. 9


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KAPLAN 5

17. Substance D decays radioactively according tofirst-order kinetics. If the initial concentraitonof Substance D is D0 and the rate constant is k(s-1), what is the concentration of D after threeminutes?

A.3k

0[D ]e

B.180k

0[D ]e

C.

k

0[D ]

3

e

D.60k

03[D ]e

18. The transition state for a given reaction:

A. has greater energy than either the

reactants or the products.

B. is an intermediate in the formation of a

product, and hence has a slightly lower

energy than the reactants but a higherenergy than the products.

C. can usually be isolated before a product is

formed.

D. is only theoretical and can never be

measured.

19. For a given exothermic reaction, how are H,Ea (forward) and Ea (reverse) related?

A. H + Ea (forward) = Ea (reverse)

B. H - Ea (forward) = Ea (reverse)

C. H = Ea (forward) + Ea (reverse)

D. H = Ea (forward) - Ea (reverse)

20. Which of the following does NOT affect therate of a reaction?

A. Temperature

B. Presence of a catalyst

C. Quantity of reactants

D. Concentration of reactants

21. All of the following are state functionsEXCEPT:

A. Volume

B. Entropy

C. Work

D. Enthalpy

22. Under what conditions does a real gas differ inbehavior from an ideal gas?

A. increased pressure only

B. increased temperature only

C. increased pressure and decreased

temperature

D. only when the molar volume is very large

23. A chemist adds 125 mL of water to 250 mL ofa 2 M solution of sulfuric acid. The newconcentration is:

A. 1 M

B. 1.33 M

C. 1.67 M

D. unchanged, because all the protons remain

in solution.

24. In a 0.75 M solution of acetic acid, the protonconcentration is:

A. greater than 0.75 M because acetic acid is

a strong acid.

B. greater than 0.75 M because acetic acid

has more than one proton available for

dissociation.

C. 0.75 M because all of the protons

dissociate for a weak acid.

D. less than 0.75 M because acetic acid is a

weak acid.

25. Which of the following has the highest molarsolubility?

A. CaCO3 (Ksp = 8.7 10-9)

B. BaSO4 (Ksp = 1.1 10-10)

C. Ag2CO3 (Ksp = 6.2 10-12)

D. CuI (Ksp = 5.1 10-12)

26. A compound was analyzed and found tocontain 72.0 g carbon, 12.0 g hydrogen and64.0 g oxygen. What is the empirical formulafor this compound?

A. CH2O

B. C2H3O

C. C3H6O2D. C6H12O4


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30. What is the characteristic valence-electronconfiguration of the chalcogen group?

27. A solution of HCl(aq) has a pH of 4.0. Whatis the H3O

+(aq) concentration?

A. 1 10-4

B. 3 10-5

C. 2 10-6

D. 1 10-8

28. If a reaction is spontaneous and endothermic

at 25C, the:

A. Gibbs free energy is positive.

B. entropy change is positive.

C. entropy change is negative.

D. enthalpy change is negative.

29. Consider the following equilibrium in a closedsystem.

5 2 3PCl ( ) Cl ( ) PCl ( ) H 88 kJ/molg g g+ =U

Which of the following will favor the

formation of PCl5?

A. increasing the volume

B. decreasing the concentration of chlorine

gas

C. decreasing the pressure at constant

temperature

D. decreasing the temperature

A. ns2

B. ns2np

1

C. ns2np

4

D. ns1np

3


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KAPLAN 7

NO TEST MATERIAL ON THIS PAGE


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MCAT

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GENERAL CHEMISTRY DISCRETES TEST 2 ANSWER KEY

1. A

2. A

3. C

4. D

5. C

6. B

7. C

8. D

9. B

10. A

11. C

12. B

13. A

14. B

15. B

16. D

17. B

18. A

19. A

20. C

21. C

22. C

23. B

24. D

25. C

26. C

27. A

28. B

29. D

30. C


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KAPLAN 9

1. A

To calculate formal charge, use the formula:

formalcharge = group number - number of electrons in lone pairs

1- (numberof electrons in bondingpairs)

2

The group number for carbon is 4. There are no electrons in lone pairs on carbon, andthere are eight electrons in bonding pairs around carbon. Therefore, the formal charge oncarbon is:

14 (0) (8) 0

2 =

2. A

The MCAT expects you to know the names and values of a variety of concepts andquantities. This is a prime example of the need for a bit of memorization in your MCATpreparation. The principal quantum number denotes the period in which an element is found.

For a given principal quantum number, n, a range of values is possible for the Azimuthalquantum number. This range is from 0 to n-1. In case you forget it on test day, just think of aparticular period, say n=3. For n=3, electrons can be placed in s orbitals, p orbitals or dorbitals. An s orbital has Azimuthal quantum number 0, a p orbital has Azimuthal quantumnumber 1 and a d orbital has Azimuthal quantum number 2. Therefore, the range is 0 to 2, or 0to n-1.

3. C

The MCAT expects you to know the term isoelectronic. It means having the sameconfiguration of electrons. Once aluminum has lost three electrons, it has the electronicconfiguration of neon, so you should look for another species that also looks like neon. When

silicon loses an electron, it looks like elemental aluminum, not the ionized form- get rid of (A).When boron loses three electrons, it looks like helium, so throw out (B). When oxygen gainstwo electrons, it looks like neon, so the answer is (C). Just to cover your bases, check thatchlorine, when it gains one electron, looks like argon, not neon, so (D) doesnt work; again, theanswer is (C).

4. D

When examining a phase diagram, its important to keep in a mind a few definitions. Bytest day, you should know the definition of the critical point; the point at which the differencesbetween the properties of the liquid and gas phases disappear. This point appears in the upperright of a phase diagram, where temperature and pressure are both high. Other terms youshould know include the triple point, fusion, vaporization, sublimation, condensation,deposition and freezing.


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5. C

A Lewis base has a pair of electrons available for bonding. Once you draw the structures

for these compounds, it is clear that only boron trifluoride is not a Lewis base:

N

H

H H

Cl- O

B

F

F F

The answer is (C).

6. B

Each Bronsted-Lowry acid-base equilibrium can be pictured as follows: acid1 + base2conjugate acid

2+ conjugate base

1. HClO

4is an acid, so when it loses its proton, the new

species will be its conjugate base. The conjugate acid of H2O is H3O+.

7. C

This question asks you to remember the order of orbital filling for electrons. After 4d isfull, the next electron will go into a 5p orbital. If you forget the order of filling, just drawyourself a diagram like the one below.

1s

2s 2p

3s 3p 3d

4s 4p 4d 4f

5s 5p 5d 5f

6s 6p 6d 6f

7s 7p 7d 7f

8. D

The MCAT expects you to know that Ksp is a function of the structure of the salt and the

temperature of the solution. Adding a stress to the system will not affect the Ksp if thetemperature remains the same, so we can immediately cancel out answer choices (A) and (C).Increasing the solutions basicity will disrupt the equilibrium between sulfate and its conjugateacid leading to an increase in sulfate ion concentration: HSO4

-+ OH

- SO4

2-+ H2O. An

increase in sulfate ion concentration will cause precipitation of sodium sulfate via the commonion effect. Thus answer choice (D) is the correct answer.


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KAPLAN 11

9. B

As you move across a period, electronegativity increases. This is because the number ofprotons in the nucleus increases, but the shielding effect of other electrons does not increasebecause they are in the same subshell. Ionization energy and electron affinity increase forsimilar reasons, but the atomic radius does not increase across a period. Atomic radius varies

greatly across a period, but elemental sodium has a much larger radius than elemental chlorine,so atomic radius does not increase across a period. Hence, (B) is the answer youre lookingfor.

10. A

For each of the answer choices, ask yourself whether the reaction fits into the definition ofthe reaction type. (D) must be wrong, since the reaction is a formation reaction: a reactionforming a product from elements in their standard states. (C) must also be incorrect becausethe oxidation states of calcium and fluorine change from reactants to products, so the reactionis an example of oxidation-reduction. It is also a combination reactions, so (B) is incorrect.Although a solid is formed in the reaction, it is not formed from dissolved species, so thereaction is not an example of a precipitation reaction. Therefore, (A) is correct.

11. C

This question tests your knowledge of Le Chateliers principle. When a stress is appliedto a system at equilibrium, the system will shift to alleviate the stress. When the given reactionvessel is compressed (a decrease in volume) the system will shift towards the side of theequilibrium where fewer gas molecules appear; gases occupy a much greater volume thansolids or liquids. Therefore, when the given reaction is compressed, the formation of reactantswill be favored. This is choice (C).

12. B

The speed of a gas at a given temperature is given by 21

2

mv . Therefore, if one gas has

twice the mass of the other, the ratio of their molecular speeds may be calculated as follows:

2 2 2

1 2 1

160 30 ;

2v v v= = 22v . Take the square root of both sides to obtain: 1

1

2v = 2v . This is

choice (B).

13. A

Before you look at the answer choices, you should focus on the phase diagram given toyou. When the MCAT asks you to examine a diagram for information, take note of how thediagram differs or is similar to those youve seen before. The key difference between thephase diagram for Substance R and those of most substances is that the solid-liquid interface

slopes back instead up and to the right. This means that Substance R is denser in the liquidphase than in the solid phase. Substance R has strong intermolecular bonds in the liquid phase!If you noticed that the phase diagram resembles that of water, great! You know that water hasstrong intermolecular bonds in the liquid phase- these include hydrogen bonds. You could alsoanswer this question by a process of elimination: B makes no sense because there is no way totell from the phase diagram what the molar mass of a substance is. (C) is not true because thegas phase is actually less dense than the liquid phase. (D) is incorrect because it takes energyto convert from liquid to gas, so vaporization is endothermic. Again, the answer is (A).


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14. B

The partial pressure of a gas in a closed container is equal to the mole fraction of that gasmultiplied by the total pressure in the container. The total pressure in the container is given as

3 atm. The mole fraction is of Gas C is:3mol 3 1

1mol+1mol+3 mol+4 mol 9 3= = . Therefore, the

partial pressure of Gas C is: 1 3atm=1atm3

, choice (B).

15. B

The MCAT expects you to know the value of Avagadros number- the number of particlesin a mole, 6.022 x 10

23. Two moles contain twice this number, choice (B). Dont get thrown

off by the exponent; when you double a quantity, you dont multiply the exponent by two. Theanswer is (B).

16. D

When balancing equations, youll want to start with the element that appears in the least

number of species. Copper (Cu) appears the least, so balance that first. For every coppersulfide, there will be two copper iron sulfides. For every iron oxide, there will be three copperiron sulfides. The least common multiple of 2 and 3 is six, so place a six in front of the copperiron sulfide, a three in front of copper sulfide and a 2 in front of the iron oxide. Now you have:

. The next step is to balance the

number of sulfur atoms on each side of the equation. There are 12 sulfur atoms from thecopper iron sulfide, and three are used to make copper sulfide. Therefore, nine must remainfor the sulfur dioxide. Check to see that this works for the number of oxygen atoms, and youllfind that the balanced equation is:

2 2 2 3 4 26CuFeS ( ) ?O ( ) 3Cu S( ) 2Fe O ( ) ?SO ( )s g s s+ + + g

g . Therefore, the coefficient of

sulfur dioxide is 9.

2 2 2 3 4 26CuFeS ( ) 13O ( ) 3Cu S( ) 2 Fe O ( ) 9SO ( )s g s s+ + +

17. B

First-order kinetics are often observed for radioactive decay. The concentration of a

radioactive substance such as D at any time t can be expressed as . Keep in mind

that the exponent must be dimensionless in order for the equation to make any sense; payattention to the units of the rate constant when calculating the answer. The MCAT loves to tripyou up on units. The rate constant given in the question stem has units of inverse seconds, butthe time is given in minutes. You need to convert the minutes to seconds. Three minutes isequal to 180 seconds, so place 180 in the exponent next to k. Now it is clear that the answer is(B).

-kt

t 0[D ]=[D ]e

18. A

The MCAT expects you to know certain things about reactions and reaction mechanisms.

One of those things is the definition of a transition state. The transition state, also called theactivated complex, is a species that may or may not resemble molecules you normally see inyour textbook. It is often a conglomerate of the reacting species. The important thing torecognize about the transition state is that it is a high-energy species, having greater energythan either the reactants or the products, so (A) is correct. The transition state is NOT anintermediate; an intermediate is a discrete species that can be isolated. Therefore, (B) is


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KAPLAN 13

incorrect. (C) is incorrect because a transition state is usually consumed as quickly as it isformed (since its energy is so high) so that isolating it becomes difficult. (D) is incorrectbecause the transition state is an experimental species and can sometimes be observed usingsophisticated chemical techniques. The key to this question is that the transition state hasunique energetic qualities, so the answer is (A).

19. A

The activation energy for the forward reaction is the amount of energy is takes toovercome the barrier between the products and the reactants. Hence, the activation energy forthe reverse reaction is the amount of energy it takes to overcome the barrier between thereactant and products. The difference between the two is the enthalpy change for the reaction.

This can be written as Ea (reverse) - Ea (forward) = H, orH + Ea (forward) = Ea (reverse),choice (A).

20. C

Four factors affect reaction rate: reactant concentrations, temperature, medium andcatalysts. The quantity of reactants does not affect reaction rate; hence, choice (C) is thecorrect answer. Choice (A) is incorrect because reaction rate is affected by temperature. If thetemperature of a reaction is increased, the fraction of molecules with kinetic energy sufficientto overcome the activation barrier will increase, and the rate of the reaction will increase.Choice (B) is incorrect because the presence of a catalyst increases the rate of a reaction bylowering the activation energy barrier for the reaction. Once the barrier has been lowered, agreater fraction of molecules will have energy sufficient to overcome it and the rate willincrease. Finally, choice (D) is incorrect because the concentration affects reaction rate. Whenthe concentration of reactants is increased, the number of collisions between moleculesincreases, so there are more possibilities for reaction to occur, thereby increasing the reactionrate.

21. C

When the magnitude of a quantity depends only on the initial and final states of thesystem, it is known as a state function. Volume is measurable at any point in time, and doesnot depend on the path taken to get to that point, so it is a state function. Therefore, choice (A)is not the one youre looking for. The entropy of a system is not measured directly, but itdepends only on the final and initial states, so it is also a state function; B is incorrect. Workdepends on the path taken. Choice (C) is correct. Enthalpy is also a state function because itdepends only on the initial and final states of the system.

22. C

Real gases deviate from the ideal gas law due to elevated pressures and decreasedtemperatures. This is because as pressure increases, the molecules are forced closer together,so intermolecular interactions become important. When the molecules are far apart, theintermolecular forces are minimal. As temperature decreases, gas particles move more slowly,so as they pass each other, the period of time in which they are in close proximity increases.This allows more opportunities for the molecules to affect one another. Hence, deviationsfrom ideal conditions result. Choice (C) is therefore the best answer. Choice (D) is not onlyincorrect; it is an extreme answer choice because of the words only and very. Extremeanswer choices are rarely correct on the MCAT, so when you see one, even if it seems correct,


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MCAT

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examine it closely to be sure. In this case, the molar volume is not a factor in determiningwhether a gas will deviate from ideality. Again, choice (C) is correct.

23. B

To make your life easier on the MCAT, remember the following equation for dilutions:MiVi = MfVf. You already know the initial concentration (Mi=2 M) and the initial volume(Vi=0.250 L) To calculate the final volume, V

f, simply combine the added water to the initial

volume: 0.250 L + 0.125 L= 0.375 L. Then, solve for Mf: (2 M)(0.250 L) = Mf (0.375 L). Mf= 1.33 M. This is choice (B).

24. D

On the MCAT, it is important for you to recognize which acids are weak and which arestrong. Acetic acid is a commonly used MCAT example of a weak acid. In a weak acidsolution, some of the acid molecules dissociate, while others remain intact. Since some of themolecules do not dissociate, the proton concentration is less than 0.75 M. If acetic acid were astrong acid, the concentration would be 0.75 M (choice (C)). Choice (B) is incorrect becauseacetic acid only has one proton available for dissociation, and choice (A) is incorrect becauseacetic acid is not a strong acid. The correct answer is (D).

25. C

There are two things to consider for this question: the absolute value of the Ksp, and thenumber of ions dissociating for each species. As it turns out, the number of ions dissociatingfor each ionic compound will turn out to be crucial in determining the correct answer. Silvercarbonate dissociates into three ions: two silver ions and one carbonate ion. Call the initialconcentration of silver carbonate x, so that you have 2x and x for silver and carbonate

respectively. This is equal to 4x3. Set this equal to the Ksp to obtain 4x

3= 6.2 10

-12; x=

(something)10-4

. The only other answer choice that comes close is A; the molar solubility of

calcium carbonate is 9.3 10-5

. This is not quite large enough to compete with silvercarbonate; therefore, the answer is (C).

26. C

The MCAT expects you to know how to use data about the mass of a compound in orderto determine its formula. Divide each of the masses given by the molar mass for that element.

From this, youll find that72.0gC

=6molC12.0g/mol

;12.0gH

=12molH1.0g H/mol

; and

64.0gO=4molO

16.0g/molO. You now know that the formula for the compound is C6H12O4.

However, this formula is the absolute amount of each kind of atom, whereas you are lookingfor the empirical formula- the relative numbers of each kind of atom. In order to obtain that,just divide all of the subscripts you just found by two to get C3H6O2.

27. B

To calculate the hydronium ion concentration, use the following formula:pH = - log [H3O

+]. Now plug in the value given for the pH: 4.0 = -log [H3O

+]. To

eliminate the logarithm, multiply each side by (-1), then place both sides of the equation in the

exponent of 10 as follows: 3log[ H O ]-pH10 10

+

= . Now youre left with 10-4.0

= [H3O+], choice (A).


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KAPLAN 15

28. B

The question stem tells you that the reaction is spontaneous. From this, you know that the

Gibbs free energy, G, is negative. Eliminate choice (A). The question stem also tells you

that the reaction is endothermic; H is positive. Eliminate (D). Therefore, the quantity TS

must be negative because G = H - TS. The entropy change is positive, choice (B).

29. DThere are two key elements to the equilibrium equation in the question stem. First, the

reaction as written is endothermic. Second, there are two moles of gas on the product side ofthe reaction, and only one mole of gas on the left. These two considerations will help youdecide how to favor the formation of PCl5. Increasing the volume of the container will favorthe formation of the products because this will keep pressure constant; the greater number ofgaseous particles on the right is favored by a greater volume. Therefore, choice (A) isincorrect. Decreasing the concentration of chlorine gas will also favor the products; the systemwill respond to such a stress by forming more chlorine molecules, as dictated by Le Chateliersprinciple. (Le Chateliers principle is an MCAT favorite.) This means that choice (B) isincorrect. Decreasing the pressure at constant temperature will also favor the products, for the

same reasoning as choice (A), so (C) is incorrect. Only lowering the temperature will favor theformation of the reactant, so choice (D) is correct.

30. C

In order to answer this question correctly, you must know the term chalcogen and youmust be able to give the general valence-electron configuration for a given group of elements.The chalcogen group is directly to the left of the halogen group; it includes oxygen and sulfur-elements you have encountered many times in your studies of general chemistry. Theseelements have filled s orbitals, ns

2and four electrons in p orbitals, np

4. The overall

configuration is ns2 np4, choice (C).

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Chemistry Discrete Test 2