CHEM 123                                                                                          Exam 3

Mark your answers directly on the Scantron Sheet in Pencil.. If your change your answer be sure to thoroughly erase the previous mark.

 

1.

At the equivalence point in an acid-base titration 

 

A.  

the [H3O+] equals the Ka of the acid.

 

B.  

the [H3O+] equals the Ka of the indicator.

 

C.  

the amounts of acid and base which have been combined are in their stoichiometric ratio.

 

D.  

the pH is 7.0.

 

E.  

the pH has reached a maximum.

 

2. 

When a strong acid is titrated with a strong base, the pH at the equivalence point 

 

A.  

is greater than 7.0.

 

B.  

is equal to 7.0.

 

C.  

is less than 7.0, but is not 3.5.

 

D.  

is equal to the pKa of the acid.

 

 

E.  

is equal to 3.5.

 

3. 

When a weak acid is titrated with a strong base, the pH at the equivalence point 

 

A.  

is greater than 7.0.

 

B.  

is equal to 7.0.

 

C.  

is less than 7.0.

 

D.  

is equal to the pKa of the acid.

 

 

E.  

is equal to 14.0 - pKb , where pKb is that of the base.

 

 

 

 

4. 

A 50.0-mL sample of 0.50 M HCl is titrated with 0.50 M NaOH. What is the pH of the solution after 28.0 mL of NaOH have been added to the acid? 

 

A.  

0.85

 

B.  

0.75

 

C.  

0.66

 

D.  

0.49

 

E.  

3.8

 

5. 

A 20.0-mL sample of 0.30 M HClO was titrated with 0.30 M NaOH. The following data were collected during the titration.

mL NaOH added                                      5.00         10.00            15.00            20.00

pH                                                            6.98         7.46              7.93              10.31


What is the Ka for HClO? 

 

A.  

1.1 × 10-7

 

B.  

3.5 × 10-8

 

C.  

1.2 × 10-8

 

D.  

4.9 × 10-11

 

E.  

none of these choices is correct

 

 

 

 

6. 

A change in pH will significantly affect the solubility of which, if any, of the following compounds? 

 

A.  

BaF2

 

B.  

CuCl

 

C.  

CuBr

 

D.  

AgI

 

E.  

None of the solubilities will be significantly affected.

 

7. 

Write the ion product expression for magnesium fluoride, MgF2. 

 

A.  

Picture

 

B.  

Picture

 

C.  

Picture

 

D.  

Picture

 

E.  

Picture

 

8. 

Calculate the molar solubility of barium carbonate, BaCO3, in pure water. Ksp = 2.0 × 10-9 

 

A.  

1.3 × 10-3 M

 

B.  

3.2 × 10-5 M

 

C.  

2.2 × 10-5 M

 

D.  

4.5 × 10-5 M

 

E.  

4.0 × 10-18 M

 

9. 

Calculate the solubility of lead(II) iodide, PbI2, in 0.025 M KI. Ksp = 7.9 × 10-9 

 

A.  

4.5 × 10-2 M

 

B.  

2.8 × 10-2 M

 

C.  

8.9 × 10-5 M

 

D.  

5.0 × 10-5 M

 

E.  

1.3 × 10-5 M

 

 

10. 

The concentration of the complex ion in each of following solutions is 1.00 M. In which of the solutions will the concentration of the uncomplexed metal ion be the greatest?

Hg(CN)42- Kf = 9.3 × 1038
Be(OH)42- Kf = 4.0 × 1018
Zn(OH)42- Kf = 3.0 × 1015
Cu(NH3)42+ Kf = 5.6 × 1011
CdI42- Kf = 1.0 × 106 

 

A.  

Hg2+

 

B.  

Be2+

 

C.  

Zn2+

 

D.  

Cu2+

 

E.  

Cd2+

 

11. 

Which relationship or statement best describes ΔS° for the following reaction?

Pb(s) + Cl2(g) → PbCl2(s) 

 

A.  

ΔS° ≈ 0

 

B.  

ΔS° < 0

 

C.  

ΔS° > 0

 

D.  

ΔS° = ΔH°/T

 

 

12. 

Which of the following pairs has the member with the greater molar entropy listed first? All systems are at 25°C. 

 

A.  

CO(g), CO2(g)

 

B.  

NaCl(s), NaCl(aq)

 

C.  

H2S(g), H2S(aq)

 

D.  

Li(s), Pb(s)

 

E.  

H2(g), H2O(g)

 

13. 

Calculate ΔS°for the reaction

SiCl4(g) + 2Mg(s) → 2MgCl2(s) + Si(s)

Substance:                           SiCl4(g)                Mg(s)            MgCl2(s)                  Si(s)

S°(J/K·mol):                         330.73                 32.68            89.62                       18.83

 

 

A.  

-254.96 J/K

 

B.  

-198.02 J/K

 

C.  

198.02 J/K

 

D.  

254.96 J/K

 

E.  

471.86 J/K

 

14. 

For a chemical reaction to be spontaneous only at high temperatures, which of the following conditions must be met? 

 

A.  

ΔS° > 0, ΔH° > 0

 

B.  

ΔS° > 0, ΔH° < 0

 

C.  

ΔS° < 0, ΔH° < 0

 

D.  

ΔS° < 0, ΔH° > 0

 

 

E.  

ΔG° > 0

 

 

 

15. 

Calculate ΔG° for the reaction

SiCl4(g) + 2Mg(s) → 2MgCl2(s) + Si(s)

Substance:                                   SiCl4(g)               Mg(s)           MgCl2(s)                 Si(s)

ΔG° f (kJ/mol):                            -616.98               0                  -591.79                  0

 

 

A.  

566.60 kJ

 

B.  

50.38 kJ

 

C.  

25.19 kJ

 

D.  

-25.19 kJ

 

E.  

-566.60 kJ

 

16. 

Hydrogen sulfide decomposes according to the following reaction

2H2S(g) → 2H2(g) + S2(g)

For this reaction at 298K ΔS° = 78.1 J/K, ΔH° = 169.4 kJ, and ΔG° = 146.1 kJ. What is the value of ΔG° at 900 K? 

 

A.  

-69881 kJ

 

B.  

48.4 kJ

 

C.  

99.1 kJ

 

D.  

240 kJ

 

E.  

441 kJ

 

 

 

17. 

Iron(III) oxide can be reduced by carbon monoxide.

Fe2O3(s) + 3CO(g)  Picture 2Fe(s) + 3CO2(g)

Use the following thermodynamic data at 298 K to determine the equilibrium constant at this temperature.

Substance:                                   Fe2O3(s)               CO(g)           Fe(s)           CO2(g)

ΔH° f (kJ/mol):                             -824.2                  -110.5           0                 -393.5

ΔG° f (kJ/mol):                             -742.2                  -137.2           0                 -394.4

S°(J/K·mol):                                 87.4                      197.7             27.78          213.7

 

 

A.  

7.0 × 10-6

 

B.  

1.3 × 10-3

 

C.  

2.2 × 104

 

D.  

1.4 × 105

 

E.  

> 2.0 × 105

 

18. 

Consider the following balanced redox reaction

3CuO(s) + 2NH3(aq) → N2(g) + 3H2O(l) + 3Cu(s)

Which of the following statements is true? 

 

A.  

CuO(s) is the oxidizing agent and copper is reduced.

 

B.  

CuO(s) is the oxidizing agent and copper is oxidized.

 

C.  

CuO(s) is the reducing agent and copper is oxidized.

 

D.  

CuO(s) is the reducing agent and copper is reduced.

 

E.  

CuO(s) is the oxidizing agent and N2(g) is the reducing agent.

 

19. 

When the following redox equation is balanced with smallest whole number coefficients, the coefficient for zinc will be _____.

Zn(s) + ReO4-(aq) → Re(s) + Zn2+(aq) (acidic solution) 

 

A.  

2

 

B.  

7

 

C.  

8

 

D.  

16

 

E.  

none of these choices is correct

 

 

20. 

Which of the following statements about voltaic and electrolytic cells is correct? 

 

A.  

The anode will definitely gain weight in a voltaic cell.

 

B.  

Oxidation occurs at the cathode of both cells.

 

C.  

The free energy change, ΔG, is negative for the voltaic cell.

 

D.  

The electrons in the external wire flow from cathode to anode in an electrolytic cell.

 

E.  

None of these statements is correct.

 

21. 

A voltaic cell is prepared using copper and silver. Its cell notation is shown below.

Cu(s) | Cu2+(aq) || Ag+(aq) | Ag(s)

Which of the following processes occurs at the cathode? 

 

A.  

Cu(s) → Cu2+(aq) + 2e-

 

B.  

Cu2+(aq) + 2e- → Cu(s)

 

C.  

Ag(s) → Ag+(aq) + e-

 

D.  

Ag+(aq) + e- → Ag(s)

 

E.  

Cu(s) + 2Ag+(aq) → Cu2+(aq) + 2Ag(s)

 

22. 

A cell can be prepared from copper and tin. What is the E°cell for the cell that forms from the following half-reactions?

Cu2+(aq) + 2e-  Picture Cu(s); E° = 0.34 V
Sn4+(aq) + 2e-  Picture Sn2+(aq); E° = 0.13 V 

 

A.  

0.47 V

 

B.  

0.21 V

 

C.  

-0.21 V

 

D.  

-0.47 V

 

E.  

0.42 V

 

 

23. 

Calculate E°cell and indicate whether the overall reaction shown is spontaneous or nonspontaneous.

Co3+(aq) + e-  Picture Co2+(aq); E° = 1.82 V
MnO4-(aq) + 2H2O(l) + 3e-  Picture MnO2(s) + 4OH-(aq); E° = 0.59 V

Overall reaction:

MnO4-(aq) + 2H2O(l) + 3Co2+(aq) → MnO2(s) + 3Co3+(aq) + 4OH-(aq) 

 

A.  

E°cell = -1.23 V, spontaneous

 

B.  

E°cell = -1.23 V, nonspontaneous

 

C.  

E°cell = 1.23 V, spontaneous

 

D.  

E°cell = 1.23 V, nonspontaneous

 

E.  

E°cell = -0.05 V, nonspontaneous

 

24. 

What is the value of the equilibrium constant for the cell reaction below at 25°C? E°cell = 0.30 V

Sn2+(aq) + Fe(s)  Picture Sn(s) + Fe2+(aq) 

 

A.  

1.2 × 105

 

B.  

1.4 × 1010

 

C.  

8.6 × 10-6

 

D.  

7.1 × 10-11

 

E.  

2.3 × 1023

 

25. 

What is the value of the equilibrium constant for the cell reaction below at 25°C? E°cell = 0.30 V

Sn2+(aq) + Fe(s)  Picture Sn(s) + Fe2+(aq) 

 

A.  

1.2 × 105

 

B.  

1.4 × 1010

 

C.  

8.6 × 10-6

 

D.  

7.1 × 10-11

 

E.  

2.3 × 1023

 

26. 

A voltaic cell consists of an Au/Au3+ electrode (E° = 1.50 V) and a Cu/Cu2+ electrode (E° = 0.34 V). Calculate [Au3+] if [Cu2+] = 1.20 M and Ecell = 1.13 V at 25°C. 

 

A.  

0.001 M

 

B.  

0.002 M

 

C.  

0.01 M

 

D.  

0.02 M

 

E.  

0.04 M