Chemistry 122
Practice problems – Exam 1
DO NOT ONLY study these problems for your EXAM. You are responsible for everything
else that was discussed in class and learned from CHM 121.
Question I: Multiple choices: Choose one best answer for each of the following question below (2pts each)
1.
2.
If Kc is the equilibrium constant for a forward reaction, 2 A⇌ B, what is Kc´ for the reaction 4 A⇌ 2B?
1
A) Kc
2
B) Kc
C) 2 Kc
D) (Kc)2
Which statement is true for a reaction with Kc equal to 2.43 × 10-12?
A)
Increasing the temperature will not change the value of Kc.
B)
There are appreciable concentrations of both reactants and products.
C)
The reaction proceeds hardly at all towards completion.
D)
The reaction proceeds nearly all the way to completion.
3.
Shown below is a concentration vs. time plot for the reaction A ⇌ 2B. For this reaction the value of the
equilibrium constant is
A) Kc < 1.
B) Kc = 0.
C) Kc = 1.
D) Kc > 1.
4.
The following pictures represent equal volumes of aqueous solutions of three acids HA (A = X, Y, or Z);
water molecules have been omitted for clarity
Arrange the acids in order of increasing value of Ka.
1
A)
Ka(HZ) < Ka(HX) < Ka(HY)
B)
Ka(HX) < Ka(HZ) < Ka(HY)
C)
Ka(HZ) < Ka(HY) < Ka(HX)
B)
Ka(HY) < Ka(HZ) < Ka(HX)
5..
A solution with a hydrogen ion concentration of 3.25 × 10-2 M is ________ and has a
hydroxide concentration of ________.
A) acidic, 3.08 × 10-12 M
-13
B) acidic, 3.08 × 10 M
-12
C) basic, 3.08 × 10 M
D) basic, 3.08 × 10-13
6.
Which of the following is a WEAK acid?
A) HClO4
B) H2SO4
C) HCl
D) HCHO2
7.
What is the conjugate base of H2PO4⁻ ?
A) HPO42B) PO43C) H3PO4
D) H3O+
E) OH⁻
Question II
A. Consider the following reaction
C(s) + 2 H2(g) ⇌ CH4(g)
ΔH° = –74.8 kJ
Does the amount of H2(g) increases, decreases or remain constant when equilibrium of reactants and products is
subjected to the following changes?
a. The temperature decreases; Also account for the change of Kc
b. The volume increases; also account for the change of Qc
c. CH4 is added
d. The catalyst is added
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B. If an equal amount of acid and base were used in the following reaction, predict which direction will the
reaction favored at equilibrium. Explain why.
…NH4+(aq)
+ CO3-2 (aq)
⇌
NH3(g) + HCO3-(aq)
C. identify the Lewis acid and the Lewis base. Explain your answer by showing a complete Dot Lewis
structure for the two reactants. (Extra Credits – 3pts: using arrow to show how electrons are moved)
NH3 +
BF3 
Base Acid
Draw Dots Lewis structure for each
D. Classify each of the following salt solutions as neutral, acidic or basic. Explain your answer by showing a
hydrolysis equation for each ion. Write NR for those that do not hydrolyze in water.
CH3NH3I (aq)
(acidic)
Question III
A. Consider the following reaction:
At a certain temperature, Kc = 8.5 x 10-3. A reaction mixture at this temperature containing solid NH4NS has [NH3]
= 0.166 M and [H2S] = 0.166. Will more of the solid form or will some of the existing solid decompose as
equilibrium is reached? Calculation must be shown to support your answer.
More solid
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B. The decomposition of ammonia is: 2 NH3(g) ⇌ N2(g) + 3 H2(g). If the partial pressure of ammonia is 1.6 × 10-3
atm and the partial pressures of N2 and H2 are each 0.25 atm at equilibrium, what is the value for Kc at 400°C for
the forward reaction?
0.50
C.
What is the percent dissociation of glycine if the solution has a pH = 8.60 and pKa = 9.60?
9.0%
Question IV
Acrylic acid (HC3H3O2) is used in the manufacture of paints and plastics. The pKa of acrylic acid is 4.25
a. Write an acid dissociation equation
b. Calculate the pH and the concentrations of all species (H3O+, C3H3O2-. HC3H3O2, and –OH) in 0.150 M acrylic acid
c. Calculate the percent dissociation in 0.0500M acrylic acid.
a. X = 0.0029
b. 2.54, 2.9 x 10-3, 2.9 x 10-3, 0.147, 4.3 x 10-12
c. c. 3.4 %
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Aniline (C6H5NH2, Kb = 4.3 x 10-10 at 25oC) is an industrially important amine used in the making of dyes.
Determine the pH of an aniline solution made by dissolving 6.90 g of aniline in enough water to make 100.0 ml of
solution.
pH = 9.25
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