AP Chemistry
Fall Semester Review
Torok
1. 1972
Consider the following melting points in degrees Celsius:
Alkali metals
Halogens
Li
181
F2
-119
Na
98
Cl2
-101
K
63
Br2
-7
Rb
39
I2
+104
Cs
29
(a) Account for the trend in the melting points of the alkali metals.
(b) Account for the trend in the melting points of the halogens.
(Make sure that your discussion clarifies the difference between the two trends.)
2. 1973 D
First ionization Energy
(kilocalories/mole)
Covalent Radii, Å
Li
124
1.34
Be
215
0.90
B
191
0.82
C
260
0.77
N
336
0.75
O
314
0.73
F
402
0.72
The covalent radii decrease regularly from Li to F, whereas the first ionization energies do not. For the ionization
energies, show how currently accepted theoretical concepts can be used to explain the general trend and the two
discontinuities.
3. 2001 B
Answer the following questions about acetylsalicylic acid, the active ingredient in aspirin.
(a) The amount of acetylsalicylic acid in a single aspirin tablet is 325 mg, yet the tablet has a mass of 2.00 g. Calculate
the mass percent of acetylsalicylic acid in the tablet.
(b) The elements contained in acetylsalicylic acid are hydrogen, carbon, and oxygen. The combustion of 3.000 g of the
pure compound yields 1.200 g of water and 3.72 L of dry carbon dioxide, measured at 750. mm Hg and 25C.
Calculate the mass, in g, of each element in the 3.000 g sample.
(c) A student dissolved 1.625 g of pure acetylsalicylic acid in distilled water and titrated the resulting solution to the
equivalence point using 88.43 mL of 0.102 M NaOH(aq). Assuming that acetylsalicylic acid has only one ionizable
hydrogen, calculate the molar mass of the acid.
(d) A 2.00  10-3 mole sample of pure acetylsalicylic acid was dissolved in 15.00 mL of water and then titrated with
0.100 M NaOH(aq). The equivalence point was reached after 20.00 mL of the NaOH solution had been added. Using
the data from the titration, shown in the table below, determine
(i) the value of the acid dissociation constant, Ka, for acetylsalicylic acid and
(ii) the pH of the solution after a total volume of 25.00 mL of the NaOH solution had been added (assume that
volumes are additive).
Volume of 0.100M
0.00
5.00
10.00
15.00
20.00
25.00
NaOH Added (mL)
pH
2.22
2.97
3.44
3.92
8.13
?
AP Chemistry
Fall Semester Review
Torok
4. 1995 B
A sample of dolomitic limestone containing only CaCO3 and MgCO3 was analyzed.
(a) When a 0.2800 gram sample of this limestone was decomposed by heating, 75.0 milliliters of CO 2 at 750 mm Hg
and 20C were evolved. How many grams of CO2 were produced.
(b) Write equations for the decomposition of both carbonates described above.
(c) It was also determined that the initial sample contained 0.0448 gram of calcium. What percent of the limestone by
mass was CaCO3?
(d) How many grams of the magnesium-containing product were present in the sample in (a) after it had been heated?
5. 1988 B
An electrochemical cell consists of a tin electrode in an acidic solution of 1.00 molar Sn2+ connected by a salt bridge to a
second compartment with a silver electrode in an acidic solution of 1.00 molar Ag+.
(a) Write the equation for the half–cell reaction occurring at each electrode. Indicate which half–reaction occurs at the
anode.
(b) Write the balanced chemical equation for the overall spontaneous cell reaction that occurs when the circuit is
complete. Calculate the standard voltage, E, for this cell reaction.
(c) Calculate the equilibrium constant for this cell reaction at 298K.
E 
Hint: n= number of moles of electrons being transferred; E= E cell;
0 . 05 92
n
log K OR
 nFE  R T ln K
(d) A cell similar to the one described above is constructed with solutions that have initial concentrations of 1.00 molar
Sn2+ and 0.0200 molar Ag+. Calculate the initial voltage, E, of this cell.
6. 1993 D
A galvanic cell is constructed using a chromium electrode in a 1.00-molar solution of Cr(NO3)3 and a copper electrode in
a 1.00-molar solution of Cu(NO3)2. Both solutions are at 25C.
(a) Write a balanced net ionic equation for the spontaneous reaction that occurs as the cell operates. Identify the
oxidizing agent and the reducing agent.
(b) A partial diagram of the cell is shown below.
Cu
Cr
1 .0 M
C r(N O 3)3
1 .0 M
C u (N O 3)3
(i) Which metal is the cathode?
(ii) What additional component is necessary to make the cell operate?
(iii) What function does the component in (ii) serve?
(c) How does the potential of this cell change if the concentration of Cr(NO3)3 is changed to 3.00-
AP Chemistry
Fall Semester Review
Torok
7. Suppose the substances in the reaction below are at equilibrium at 600K in volume V and at pressure P. State
whether the partial pressure of NH3(g) will have increased, decreased, or remained the same when equilibrium
is reestablished after each of the following disturbances of the original system. Some solid NH4Cl remains in the
flask at all times. Justify each answer with a one-or-two sentence explanation.
NH4Cl(s)  NH3(g) + HCl(g)
H = +42.1 kilocalories
(a)
(b)
(c)
(d)
(e)
A small quantity of NH4Cl is added.
The temperature of the system is increased.
The volume of the system is increased.
A quantity of gaseous HCl is added.
A quantity of gaseous NH3 is added.
8. Ammonium hydrogen sulfide is a crystalline solid that decomposes as follows:
NH4HS(s)  NH3(g) + H2S(g)
(a) Some solid NH4HS is placed in an evacuated vessel at 25ºC. After equilibrium is attained, the total pressure inside the
vessel is found to be 0.659 atmosphere. Some solid NH4HS remains in the vessel at equilibrium. For this
decomposition, write the expression for KP and calculate its numerical value at 25ºC.
(b) Some extra NH3 gas is injected into the vessel containing the sample described in part (a). When equilibrium is
reestablished at 25ºC, the partial pressure of NH3 in the vessel is twice the partial pressure of H2S. Calculate the
numerical value of the partial pressure of NH3 and the partial pressure of H2S in the vessel after the NH3 has been
added and the equilibrium has been reestablished.
(c) In a different experiment, NH3 gas and H2S gas are introduced into an empty 1.00 liter vessel at 25ºC. The initial
partial pressure of each gas is 0.500 atmospheres. Calculate the number of moles of solid NH4HS that is present
when equilibrium is established.
9. When H2(g) is mixed with CO2(g) at 2,000 K, equilibrium is achieved according to the equation below. In one
experiment, the following equilibrium concentrations were measured.
CO2(g) + H2(g)  H2O(g) + CO(g)
[H2]
= 0.20 mol/L
[CO2] = 0.30 mol/L
[H2O] = [CO]
= 0.55 mol/L
(a) What is the mole fraction of CO(g) in the equilibrium mixture?
(b) Using the equilibrium concentrations given above, calculate the value of Kc, the equilibrium constant for the
reaction.
(c) Determine Kp in terms of Kc for this system.
Hint: Δn= (total moles of gas on products side)-(total moles of gas on reactants side); ; Kp=Kc(RT)Δn
(d) When the system is cooled from 2,000 K to a lower temperature, 30.0 percent of the CO(g) is converted back to
CO2(g). Calculate the value of Kc at this lower temperature.
(e) In a different experiment, 0.50 mole of H2(g) is mixed with 0.50 mole of CO2(g) in a 3.0-liter reaction vessel at 2,000
K. Calculate the equilibrium concentration, in moles per liter, of CO(g) at this temperature.
AP Chemistry
Fall Semester Review
Torok
10. A 0.682 g sample of an unknown weak monoprotic organic acid, HA, was dissolved in sufficient water to make 50
milliliters of solution. It was titrated with 0.135 molar NaOH solution. After the addition of 10.6 milliliters of
base, a pH of 5.65 was recorded. The equivalence point (end point) was reached after addition of 27.4 milliliters
of the 0.135 molar NaOH.
a) Calculate the number of moles of acid in the original sample.
b) Calculate the molar mass of the acid, HA.
c) Calculate the number of moles of unreacted HA remaining in solution when the pH was 5.65.
d) Calculate the hydronium ion concentration at pH = 5.65
e) Calculate the value of the ionization constant, Ka, of the acid HA.
11.
CH3NH2 + H2O  CH3NH3+ + OH–
Methylamine, CH3NH2, is a weak base that reacts according to the equation above. The value of the ionization constant,
Kb, is 5.25 X 10–4. Methylamine forms salts such as methylammonium nitrate, (CH3NH3+)(NO3–).
(a) Calculate the hydroxide ion concentration, [OH–], of a 0.225–molar aqueous solution of methylamine.
(b) Calculate the pH of a solution made by adding 0.0100 mole of solid methylammonium nitrate to 120.0 milliliters
of a 0.225–molar solution of methylamine. Assume no volume change occurs.
(c) How many moles of either NaOH or HCl (state clearly which you choose) should be added to the solution in (b)
to produce a solution that has a pH of 11.00? Assume that no volume change occurs.
(d) A volume of 100. milliliters of distilled water is added to the solution in (c). How is the pH of the solution
affected? Explain.