Chem 121 Fritsch

Ch 15 and 16 Practice Problems - KEY The following problems are intended to provide you with additional practice in preparing for the exam. Questions come from the textbook, previous quizzes, previous exams, and other sources. A solutions manual is supplied in a separate document. 1. For the following reaction

2SO3(g) 2SO2(g) + O2(g), Kc = 1.3 x 10-5 at 500 K.

If [SO3] = [SO2] = [O2] = 1.75 103 M at 500 K, which one of the following statements is true? A. The system is at equilibrium, thus no concentration changes will occur. B. The concentrations of SO3 and O2 will increase as the system approaches equilibrium. C. The concentration of SO3 will increase as the system approaches equilibrium. D. The concentrations of SO2 and SO3 will fall as the system moves toward equilibrium. E. The concentrations of SO2 and O2 will increase as the system approaches equilibrium. Answer:_____C____

2. What is the equilibrium constant for the overall reaction? overall reaction SO3(g) + NO2(g) ↔ SO2(g) + NO3(g) Kc = ? elementary reaction 2 SO2(g) + O2(g) ↔ 2 SO3(g) Kc1 = 1.89 x 1013 elementary reaction NO2(g) + ½O2(g) ↔ NO3(g) Kc2 = 7.14 x 102

3. Consider two solutions of formic acid HCHO2 (pKa = 3.74) which were prepared with concentrations of 0.19 M and 0.0034 M HCHO2. a. Which has a lower pH? b. Which has a higher percentage ionization?

Chem 121 Fritsch

4. For the following solutions, determine the [H3O+], [OH-], and the pH. a. A 0.35 M HNO2 solution (pKa = 3.14)

Chem 121 Fritsch

b. A 0.35 M HNO3 solution

c. A 0.00035 M HNO2 solution

d. A 0.16 M KOH solution

Chem 121 Fritsch

e. A 0.16 M KCN solution (pKb = 4.79)

f. A 0.16 M H2SO3 solution (pKa1 = 1.89, pKa2 = 7.21)

Chem 121 Fritsch

5. Cobalt phosphate (Co3(PO4)2 , 15.38 g, 366.75 g/mol) and chromium(III) chloride (CrCl3, 3.46 g, 158.35 g/mol) were added to enough water to make a 100 mL solution. At equilibrium, [Co1+]eq = 0.0172 M.

5a. Calculate the equilibrium constant.

5b. Later, chromium(IV) oxide (CrO2, 3.678 g, 84.00 g/mol) was added to the solution. Determine the concentration of Co1+ after equilibrium was re-established. (Assume no change in solution volume with the addition of the chromium(IV) oxide.)

Chem 121 Fritsch

6. In the reaction below, identify the conjugate base of HSO4

is…. SO42 –

HSO4

(aq) + OH(aq) SO4

2(aq) + H2O(l)

7. Place the follow in order of increasing acidity (HC2H3O2, NH4Cl, HClO4, HCHO2) Increasingly strong acid NH4Cl < HC2H3O2 < HCHO2 < HClO4

Chem 121 Fritsch

8. The reaction below is at equilibrium, how will the equilibrium will shift to the left, the right, or remain unchanged when the system is modified. Place an X in the appropriate box. Use same starting equilibrium conditions for each individual change.

SO2(g) + NO3(g) SO3(g) + NO2(g) Hºrxn = -15 kJ/mol

Equilibrium will shift to the

Change to the system Left Unchanged Right

SO2 is added. X

The volume is increased. X

NO2 is added. X

The temperature is decreased. X

9. Morphine (C17H19NO3, molar mass = 285.34 g/mol) is used to manage pain and is often administered intravenously where the concentration is 1 mg morphine per mL of solution. In the General Chemistry laboratory you prepare an analogous solution from morphine and de-ionized water only. Calculate the pH of the solution you have prepared.

Chem 121 Fritsch

Chem 121 Fritsch

10. Calculate the percent ionization for 100 mL of a 0.1 M benzoic acid solution with pH = 2.50. (Benzoic acid, HC7H5O2 is a white solid with MW = 122.0 g/mol)

HC7H5O2(aq) + H2O(l) C7H5O2-(aq) + H3O+

(aq) pKa = 4.20

11. Which of the following graphs shows the correct relationship between the molarity of a weak acid and its percent ionization?

A B C D

Answer:_____C____

12. For the following chemical equations, balance the equations and then write the equilibrium expression (Kc) and the reaction quotient (Q). a. NO(g) + O2(g) ↔ N2O3(g) b. SF6(g) + SO3(g) ↔ SO2F2(g) c. C2H6(g) + O2(g) ↔ CO2(g) + H2O(g)

Chem 121 Fritsch

13. In a 5.0 L flask, 0.25 atm HBr(g), 0.15 atm H2(g), and 0.025 atm Br2(g) were combined. The temperature was raised to 425 °C and allowed to come to equililbrium. What are the equilibrium partial pressures of HBr, H2, and Br2?

2HBr(g) ↔ H2(g) + Br2(g) Kp = 4.18 x 10-9 at 425 °C

14. The formation of NO(g) from its elements is not favored Kc = 4.10 x 10-3 at 2000 °C. What will be the equilibrium concentrations of N2 and O2 after 0.264 mol of NO are added to a 1.5L flask and the temperature raised to 2000 °C? What are the masses in grams of N2 and NO in the flask at equilibrium?

N2(g) + O2(g) ↔ 2NO(g)

Chem 121 Fritsch

15. During the combustion of H2, hydroxyl radical (•OH) is formed by the reaction,

H(g) + ½O2(g) ↔ •OH(g) Kc = ? Use the following reactions to determine Kc for the above reaction.

½H2(g) + ½O2(g) ↔ •OH(g) Kc = 0.58 H2(g) ↔ 2H(g) Kc = 1.6 x 10-3

16. Write Ka expressions for the following: a. HCN b. HCO3

- c. HBrO2 d. H3PO3

Chem 121 Fritsch

17. Choose which solution will have a lower pH when 0.15 moles of following solutes are is dissolved in enough water to make a 250 mL solution. a. H3PO4 or H3AsO4 b. HBrO or HBrO2 c. Co(NO3)2 or Co(NO3)3 d. FeCl3 or AlCl3

18. Identify the Lewis acid and Lewis Base among the reactants in each of the following reactions. a. Ag+ + 2NH3 ↔ [Ag(NH3)2]+ b. AlCl3 + Cl- ↔ [AlCl4]- c. CN- + H2O ↔ HCN + OH- d. Ni2+ + 4CO ↔ [Ni(CO)4]2+ e. H2SO4 + NH3 ↔ HSO4

- + NH4+

Chem 121 Fritsch

19. Strong acids like HCl fully dissociate into ions in aqueous solution. Can a weak acid like acetic acid (pKa = 4.74) dissociate to greater than 90 % in aqueous solution? A. Yes at any concentration. B. Yes only at low concentration (<1 x 10-2 M). C. Yes only at high concentration (>1 M). D. No, weak acids do not fully dissociate at any concentration. Answer:____B_____

20. Like water, other compounds undergo autoionization. For methanol (CH3OH), the autoionization constant (Kmet) is 2.0 x10-17 at 25 °C. Calculate the [CH3O-] and [CH3OH2

+] in pure methanol at 25 °C.

Chem 121 Fritsch

21. Dinitrogen tetraoxide (N2O4, 32.56 g, 92.01 g/mol) was added to a 10 L flask and the system was allowed to come to equilibrium according to the reaction below at 430 °C. At equilibrium [NO2]eq = 0.0117 M.

21a. Calculate the equilibrium constant.

21b. The gas was compressed to ½ its original volume with no temperature change. Find the [N2O4]eq after compression.

Chem 121 Fritsch

22. For each of the following identify the acid, base, conjugate acid, and conjugate base. a. HC7H4NO4(aq) + H2O(l) ↔C7H4NO4

-(aq) + H3O+

(aq) b. NH4Cl(aq) + H2O(l) ↔NH3(aq) + H3O+

(aq) + Cl-(aq)

c. KCN(aq) + HCHO2(aq) ↔HCN(aq) + K+(aq) + CHO2

-(aq)

Chem 121 Fritsch

23. Acid strength decreases in the series HI > HSO4 > HF > HCN. Which of the following anions is the

weakest base?

A. I

B. SO42

C. F

D. CN E. It cannot be determined from the available information. Answer:____A____