Cracking the AP Chemistry Exam

be multiplied by 2 so that the electrons balance.
Additionally, a volt is equal to a Joule/Coulomb,
which is what you should use to get the units to
make sense. So:

∆G = –(4 mol e–)(96,500 C/mol e–)(–0.98 J/C)

∆G = 380,000 J or 380 kJ

(c) Which electrode in the diagram (A or B) is the cathode,
and which is the anode? Justify your answers.

Oxygen gas is evolved in the oxidation reaction, meaning
that is occuring at electrode B. Oxidation always occurs at
the anode, so electrode A is the cathode and electrode B is
the anode.

6. Aniline, C6H5NH2, is a weak base with Kb = 3.8 × 10–10.

(a) Write out the reaction that occurs when aniline reacts with
water.

The aniline will act as a proton acceptor and take a proton
from the water.

C6H5NH2(aq) + H2O(l) ↔ C6H5NH3+(aq) + OH–(aq)

(b) (i) What is the concentration of each species at
equilibrium in a solution of 0.25 M C6H5NH2?
For the above equilibrium, Kb = [C6H5NH3+]
[OH–]/[C6H5NH2]. The concentrations of both the
conjugate acid and the hydroxide ion will be equal,
and the concentration of the aniline itself will be

approximately the same, as it is a weak base which
has a very low protonation rate. You can do an ICE
chart to confirm this, but it is not really necessary if
you understand the concepts underlying weak acids
and bases.

3.8 × 10–10 = (x)(x)/(0.25)
x2 = 9.5 × 10–11
x = 9.7 × 10–6

[C6H5NH3+] = [OH–] = 9.7 × 10–6 M and
[C6H5NH2] = 0.25 M

(ii) What is the pH value for the solution in (b)(i)?

pOH = –log[OH–]
pOH = –log (9.7 × 10–6 M)
pOH = 5.01

pOH + pH = 14
5.01 + pH = 14
pH = 8.99

7. A rigid, sealed 12.00 L container is filled with 10.00 g each of
three different gases: CO2, NO, and NH3. The temperature of the
gases is held constant 35.0°C. Assume ideal behavior for all
gases.

(a) (i) What is the mole fraction of each gas?

First, the moles of each gas need calculating:

CO2 = 10.00 g × 1 mol/44.01 g = 0.227 mol
NO = 10.00 g × 1 mol/30.01 g = 0.333 mol

NH3 = 10.00g × 1 mol/17.03 g = 0.587 mol

Total moles of gas = 1.147 moles

XCO2 = 0.227/1.147 = 0.198
XNO = 0.333/1.147 = 0.290
XNH3 = 0.587/1.147 = 0.512

(ii) What is the partial pressure of each gas?

Using the Ideal Gas Law, we can calculate the total
pressure in the container:

PV = nRT
P (12.00 L) = (1.147 mol)(0.0821 atm×L/mol×K)
(308 K)
P = 2.42 atm

The partial pressure of a gas is equal to the total
pressure time the mole fraction of that gas.

PCO2 = (2.42 atm)(0.198) = 0.479 atm
PNO = (2.42 atm)(0.290) = 0.702 atm
PNH3 = (2.42 atm)(0.512) = 1.24 atm

(b) Out of the three gases, molecules of which gas will have
the highest velocity? Why?

If all gases are at the same temperature, they have the same
amount of kinetic energy. KE has aspects of both mass and
velocity, so the gas with the lowest mass would have the
highest velocity. Thus, the NH3 molecules have the highest
velocity.

(c) Name one circumstance in which the gases might deviate

from ideal behavior, and clearly explain the reason for the
deviation.

The most common reason for deviation from ideal behavior
is that the intermolecular forces of the gas molecules are
acting upon each other. This would occur when the
molecules are very close together and/or moving very
slowly, so deviations would occur at high pressures and/or
low temperatures.

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