Chemistry 112 Harwood b. NH

Chemistry 112
Sample Exams (3rd Hour)
Harwood
Chapters to be covered in Exam 3: 8.1-8.7; 9.1-9.3; 11.1-11.2; 13.3-13.5; 15 (all)
Sample Exam Questions
2+
1. Determine the formal charges for each of the unique atoms in a. SO3 b. NH4
2. For the system 2 HI(g) ↔
H2(g) + I2(g), it is found that if one starts with pure HI at a concentration of
0.50 mol/L, its concentration at equilibrium is 0.10 mol/L. Calculate KC and KP for the reaction.
3. Consider the equilibrium for the following exothermic reaction
N2(g)
+ 3 H2(g) ↔ 2 NH3(g)
A mixture of these three substances reaches equilibrium at 200°C. Predict the direction in which the system
will move (right or left) to re-establish equilibrium if
a. 1 mole of H2 is removed
b. the total pressure is increased by adding H2 (volume remains constant)
c. the volume of the container is reduced
d. the temperature is raised to 300°C
4. Find the pH of solutions with the following:
+
-2
a. [H3O ] = 1.0 × 10 M
(VALID ONLY IF WE COVER THIS IN CLASS)
-5
b. [HCl] = 7.4 × 10 M
–
-5
c. [OH ] = 4.0 × 10 M
-2
d. [KOH] = 6.2 × 10 M
5. The following pictures represent aqueous solutions of three acids HA (A = X, Y, or Z); water molecules
have been omitted for clarity:
(VALID ONLY IF WE COVER THIS IN CLASS)
(a) What is the conjugate base of each acid?
(b) Arrange the three acids in order of increasing
acid strength.
(c) Which acid, if any, is a strong acid?
(d) Which acid has the smallest value of Ka?
6. The freezing point of seawater is about –1.85°C. If seawater is an aqueous solution of sodium chloride,
calculate the molality of NaCl in seawater. The Kf for water is 1.86°C/m.
7. What is the most important kind of intermolecular force present in chloroform, CHCl3?
8. Water gas, a commercial fuel, is made by the reaction of hot coke with steam:
C(s) + H2O (g) ↔
CO(g) + H2(g)
–2
When equilibrium is established at 800°C, the concentrations of CO, H2, and H2O are 4.0 × 10
–2
4.0 × 10
M,
and 1.0 × 10
–2
a. Calculate Kc.
M,
respectively.
M,
Chemistry 112
Sample Exams (3rd Hour)
Harwood
b. What will be the final concentrations of CO and H2 if enough steam is added to raise its concentration
–2
temporarily to 4.0 × 10 M (the other concentrations are not changed)?
9. Persons are medically considered to have lead poisoning if they have a concentration of greater than 10
micrograms of lead per deciliter of blood. What is this concentration in parts per billion? (Assume that
blood has a density of 1.00 g / mL.)
10. Consider the reaction N2(g) + 3 H2(g) ↔ 2 NH3(g) where the partial pressures of N2, H2, and NH3 are 1.0,
4.2, and 6.3 atm, respectively, and the temperature is 400 K. For this reaction, KP = 41 at 400 K. Is this
reaction mixture likely to form reactants, products, or is it at equilibrium?
11. What is the shape of the following molecules? Which are polar?
–
a. SO2
b. PH3
c. NO3
d. CO2
4
12. Consider the reaction CO(g) + 2 H2(g) ↔ CH3OH(g). At room temperature, KP is approximately 2 × 10
but at a higher temperature KP is substantially smaller. Which of the following is true?
a. At the higher temperature, more CH3OH(g) is produced.
b. The reaction is endothermic.
c. The reaction is exothermic.
13. The following pictures represent mixtures of A molecules (open circles) and B molecules (closed circles),
which interconvert according to the equation A ↔ B. If mixture (1) is a equilibrium, which of the other
mixtures are also at equilibrium? Explain.
Sample Exam Answers
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
11.
12.
13.
a. S = 6 - 2 - 3 = +1, O = 6 - 6 - 1 = -1 b.
Kc = 4.0
KP = 4.0
a. left
b. right
c. right
a. pH = 2.00
b. pH = 4.13
c. pH = 9.60
–
–
–
(a) X , Y , and Z
(b) HX < HZ < HY (c)
0.497 mol kg–1
dipole-dipole interactions
a. Kc = 0.16
b. [H2] = [CO] = 0.059 M
100 ppb
products
a. bent; polar b.
pyramidal; polar
c.
c
(3) Because it has the same ratio of B:A as (1)
N = 5 - 0 - 4 = +1, H = 1 - 0 - 1 = 0
d. left
d. pH = 12.79
HY (d)
HX
trigonal planar
d. linear