Chemistry 112                                          Exam 3                                                         5/03/01

Lecture 2, MWF 11:15                        Day

 

The key to the multiple-choice questions is given after the questions.

 

The multiple-choice portion of this exam has 20 questions on 4 pages totaling 85 points. Be sure to include your name and 9 digit ID number on the answer sheet. Lecture section 2 goes in column K. Blacken in all selections using a No. 2 pencil.

 

The written portion of this exam has 4 questions totaling 15 points and is to be done on the question sheet in the spaces provided. Be sure to fill in your name and 9 digit ID number.

 

Turn in both the bubble answer sheet and the hand written sheet. Keep the multiple-choice questions to check your answers against the posted key.

 

Answers to the questions will be posted on the web as soon as possible after the exam.

 

Information for your use:

K =  °C + 273

at 25 °C            KW = 1.0 x 10-14                    pH + pOH = 14

pH = -log [H3O+]             pOH = -log [OH-]                    pKa = -log Ka

Ka x Kb = Kw                         [H3O+] = Ka( [acid] / [conj base] )

Strong Acids: HCl, HBr, HI, HNO3, HClO4, H2SO4 (Assume all other acids are weak.)

Strong Bases: NaOH, KOH, Ba(OH)2

DH° = SDH°f (products) - SDH°f (reactants)

DSuniverse  = DSsystem + DSsurroundings ³ 0

qp = DH

DS = qreversible / T                      DSsurroundings = – DHsystem / T

DS° = SS°(products) - SS°(reactants)

 

1.      (5 points)  If the pH of an aqueous 0.010 M HNO2 solution is found to be 2.70, what is the value of  Ka for this acid?

 

(A) 4.1 x 10-5       (B) 3.1 x 10-5               (C) 5.0 x 10-4       (D) 4.0 x 10-4               (E) 2.0 x 10-3

 

2.       (5 points) What is the [OH-] in a 0.50 M aqueous NH3 solution? Kb for NH3 is 1.8 x 10-5.

 

(A) 3.6 x 10-5       (B) 9.0 x 10-4               (C) 4.2 x 10-3       (D) 3.0 x 10-3               (E) 5.0 x 10-1

 

Use the Ka values for the following weak acids as needed to answer questions 3 and 4.

HF   Ka = 7.2 x 10-4                                HCN   Ka = 4.0 x 10-10

 

3.      (4 points) What is the value of Kb for the equilibrium F- + H2O  «  HF + OH-  ?

 

(A) 7.2 x 10-4       (B) 1.4 x 10-5               (C) 7.2 x 10-6       (D) 3.0 x 10-9               (E) 1.4 x 10-11

 

4.      (4 points) Which of the following would give the most basic 0.10 M aqueous solution?

 

(A) NaF                 (B) NaCN        (C) NaCl             (D) HCl                    (E) NH4Cl

 

5.      (4 points) Which of the following would give an acidic aqueous solution?

 

(A) KNO3         (B) Ba(NO3)2         (C) Fe(NO3)3       (D) Na2CO3           (E) none would

 

Use the following information for H2Se as needed for questions 6 and 7 :

H2Se + H2O  «  H3O+ + HSe-             Ka1 = 1.7 x 10-4

HSe- + H2O  «  H3O+ + Se2-               Ka2 = 1.0 x 10-10

 

6.      (5 points) What is the approximate [H3O+] in a 0.10 M aqueous H2Se solution?

 

(A) 1.0 x 10-10       (B) 1.3 x 10-7              (C) 3.2 x 10-6        (D) 1.7 x 10-4               (E) 4.1 x 10-3          

 

7.      (4 points) What is the [Se2-] in a 0.10 M aqueous H2Se solution?

 

(A) 1.0 x 10-10       (B) 1.3 x 10-7              (C) 3.2 x 10-6        (D) 1.7 x 10-4               (E) 4.1 x 10-3          

 

8.      (4 points) For the HSO3- ion, Ka = 6.4 x 10-8 and Kb = 5.9 x 10-13. The pH of an aqueous solution of  NaHSO3 is expected to be

 

(A) less than 7 due to: HSO3-(aq) + H2O(l) « SO32-(aq) + H3O+(aq)

(B) equal to 7

(C) greater than 7 due to:  HSO3-(aq) + H2O(l) « H2SO3(aq) + OH-(aq)

(D) less than 7 due to:      Na+(aq) + 2H2O(l) « NaOH(aq) + H3O+(aq)

(E) greater than 7 due to:      Na+(aq) + H2O(l) « NaH(aq) + OH-(aq)

 

9.      (4 points) Which of the following when added to 1 L of a 0.10 M HCl solution would make the solution an effective buffer?

 

(A) 0.20 mol NaNO2                  (C) 0.05 mol NaF                 (E) none of them

(B) 0.10 mol NaCH3COO                     (D) 0.10 mol NaCl

 

10.   (4 points) When 1 L of pure water is added to 1 L of a 0.25M CH3COOH / 0.25M NaCH3COO buffer solution:

 

(A) the pH increases, but by less than one unit

(B) the pH increases by more than one unit

(C) the pH remains essentially constant

(D) the pH decreases, but by less than one unit

(E) the pH decreases by more than one unit

 

11.   (5 points) The solubility of PbI2 in water is 0.0013 M. What is the value of Ksp for PbI2 ?

 

(A) 2.6 x 10-3       (B) 1.6 x 10-5              (C) 1.7 x 10-6        (D) 5.1 x 10-7               (E) 8.8 x 10-9          

 

12.   (4 points) Ksp for CuBr is 4.0 x 10-8 . What is the molarity of Cu+ in a saturated solution of CuBr?

 

(A) 2.0 x 10-4       (B) 3.5 x 10-5              (C) 8.0 x 10-6        (D) 3.0 x 10-7               (E) 4.0 x 10-8          

 

13.  (4 points) If solid sodium bromide were added to the solution in problem 12, the solubility of CuBr would _____ and the value of Ksp would _____ .

 

(A) increase, increase            (C) increase, not change      (E) increase, decrease

(B) decrease, decrease           (D) decrease, not change

 

Use the following Ksp's for questions 14 and 15:

            PbCl2   Ksp = 1.6 x 10-5      PbBr2   Ksp = 6.6 x 10-6      PbF2   Ksp = 3.7 x 10-8

 

14.   (5 points) If solid Pb(NO3)2 is added slowly to a solution that is 1.0 M in all three (Cl-, Br- and F-) until the [Pb2+] is 1.0 x 10-5 M

 

(A) only a precipitate of PbCl2 will form.

(B) only a precipitate of PbBr2 will form.

(C) only a precipitate of PbF2 will form.

(D) precipitates of both PbBr2 and PbF2 will form.

(E) precipitates of PbCl2, PbBr2 and PbF2 will form.

 

15.  (5 points) What is the [Pb2+] needed to initiate precipitation of PbCl2 from a 0.40 M NaCl solution?

 

(A) 4.0 x 10-3       (B) 1.0 x 10-4              (C) 2.5 x 10-5        (D) 4.0 x 10-5               (E) 6.4 x 10-6 

 

16.  (3 points) At constant pressure, the entropy of a substance must

 

(A) decrease as its temperature increases          (D) increase in a spontaneous process

(B) increase as its temperature increases          (E) be constant during a phase change

(C) decrease in a spontaneous process

 

17.   (4 points) The heat of vaporization of water at its normal boiling point of 100 °C is 40.7 kJ/mol. What is the value of DS in J/K for the process H2O(l)  ®  H2O(g) at 100 °C, 1 atm. ?

 

(A) 15.2           (B) -109                 (C) 109             (D) -407                  (E) 407

 

18.   (4 points) Predict which one of the following reactions would have DSo > 0.

 

(A) 2HCl(g) ®  H2(g) + Cl2(g)                     (D) NH3(g) + H2S(g)  ®  NH4HS(s)

(B) 2H2O2(l)  ®  2H2O(l) + O2(g)               (E) 2Mg(s) + O2(g) ®  2MgO(s)

(C) CH4(g) + 2O2(g)  ®  CO2(g) + 2H2O(l)

 

Questions 19 and 20 refer to the following reaction and thermodynamic data at 298K.

NH3(g)  +  HCl(g)   ®   NH4Cl(g)

         DHof (kJ/mol)            -46            -92          -314

            So(J/mol K)      192           187              95

 

19.  (4 points)  What is the standard entropy change, DSo, for the reaction in J/K at 298K?

 

(A) 219            (B) -219                 (C) 284             (D) -284                  (E) 474

 

20.  (4 points) If one mole of NH4Cl(s) is formed under standard conditions, what is the entropy change for the surroundings in J/K at 298K?.

 

(A) 176            (B) -176                 (C) 284             (D) -591                  (E) 591

 

 

The following list gives the number of points for each answer, in the order ABCDE, for each question. The correct answer to each question is the one worth the most points.

1                    00530

2                    00050

3                    00004

4                    34000

5                    00400

6                    00005

7                    40000

8                    40200

9                    40000

10                00400

11                00005

12                40000

13                02040

14                00052

15                05200

16                03000

17                00400

18                04000

19                00240

20                00024