CHEM 123, Fall 2006 Exam #2

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1.	When a chemical system is at equilibrium,

A.	the concentrations of the reactants are equal to the concentrations of the products.

B.	the concentrations of the reactants and products have reached constant values.

C.	the forward and reverse reactions have stopped.

D.	the reaction quotient, Q, has reached a maximum.

E.	the reaction quotient, Q, has reached a minimum.

2.	The two equilibrium constants for the same reaction, Kc and Kp, will always equal one another when

A.	all of the reactants and products are gases.

B.	in the reaction equation, the number of moles of gaseous products equals the number of moles of gaseous reactants.

C.	in the reaction equation, the number of moles of gaseous products is greater than the number of moles of gaseous reactants.

D.	in the reaction equation, the number of moles of gaseous products is smaller than the number of moles of gaseous reactants.

E.	in the reaction equation, the total number of moles of reactants equals that of the products.

3.	The reaction quotient, Qc, for a reaction has a value of 75 while the equilibrium constant, Kc, has a value of 195. Which of the following statements is accurate?

A.	The reaction must proceed to the left to establish equilibrium.

B.	The reaction must proceed to the right to establish equilibrium.

C.	The concentrations of the products will be much smaller than the concentrations of the reactants when the system is at equilibrium.

D.	The concentrations of the products will be about the same as the concentrations of the reactants when the system is at equilibrium.

E.	None of these statements is accurate.

4.	Write the mass-action expression, Qc , for the following chemical reaction. 2Cu^2+(aq) + 4I^-(aq)   2CuI(s) + I2(aq)

A.

B.

C.

D.

E.

5.	Consider the reactions of cadmium with the thiosulfate anion. Cd^2+(aq) + S2O3^2-(aq)   Cd(S2O3)(aq); K1 = 8.3 × 10^3 Cd(S2O3)(aq) + S2O3^2-(aq)   Cd(S2O3)2^2-(aq); K2 = 2.5 × 10^2 What is the value for the equilibrium constant for the following reaction? Cd^2+(aq) + 2S2O3^2-(aq)   Cd(S2O3)2^2-(aq)

A.

0.030

B.

33

C.	8.1 × 10^3

D.	8.6 × 10^3

E.	2.1 × 10^6

6.	The equilibrium constant, Kp , for the reaction H2(g) + I2(g)   2HI(g) is 55.2 at 425°C. A rigid cylinder at that temperature contains 0.127 atm of hydrogen, 0.134 atm of iodine, and 1.055 atm of hydrogen iodide. Is the system at equilibrium?

A.

Yes.

B.	No, the forward reaction must proceed to establish equilibrium.

C.	No, the reverse reaction must proceed to establish equilibrium.

D.	Need to know the volume of the container before deciding.

E.	Need to know the starting concentrations of all substances before deciding.

7.	Nitric oxide and bromine were allowed to react in a sealed container. When equilibrium was reached the following partial pressures of three gases were measured: NO: 0.526 atm; Br2: 1.59 atm; NOBr: 7.68 atm. Calculate Kp for the reaction. 2NO(g) + Br2(g) 2NOBr(g)

A.	7.45 × 10^-3

B.

0.109

C.

9.18

D.

91.8

E.

134

8.	At 25°C, the equilibrium constant Kc for the reaction 2A(aq)   B(aq) + C(aq) is 65. If 2.50 mol of A is added to enough water to prepare 1.00 L of solution, what will the equilibrium concentration of A be?

A.

0.038 M

B.

0.14 M

C.

0.28 M

D.

1.18 M

E.

2.4 M

9.	Ammonium iodide dissociates reversibly to ammonia and hydrogen iodide. NH4I(s)   NH3(g) + HI(g) At 400°C, Kp = 0.215. Calculate the partial pressure of ammonia at equilibrium when a sufficient quantity of ammonium iodide is heated to 400°C.

A.

0.103 atm

B.

0.215 atm

C.

0.232 atm

D.

0.464 atm

E.

2.00 atm

10.	The reaction system CS2(g) + 4H2(g)   CH4(g) + 2H2S(g) is at equilibrium. Which of the following statements describes the behavior of the system if the partial pressure of hydrogen is doubled?

A.	As equilibrium is reestablished, the partial pressure of carbon disulfide increases.

B.	As equilibrium is reestablished, the partial pressure of methane, CH4, decreases.

C.	As equilibrium is reestablished, the partial pressure of hydrogen decreases.

D.	As equilibrium is reestablished, the partial pressure of hydrogen sulfide decreases.

E.	As equilibrium is reestablished, all the partial pressures will decrease.

11.

Magnesium hydroxide is used in several antacid formulations. When it is added to water it dissociates into magnesium and hydroxide ions. Mg(OH)2(s)    Mg^2+(aq) + 2OH^-(aq) The equilibrium constant at 25°C is 8.9 × 10^-12. One hundred grams of magnesium hydroxide is added to 1.00 L of water and equilibrium is established. What happens to the solution if another 10 grams of Mg(OH)2 are now added to the mixture?

A.	The hydroxide ion concentration will decrease.

B.	The hydroxide ion concentration will increase.

C.	The hydroxide ion concentration will be unchanged.

D.	The solution will become supersaturated.

E.	None of these conclusions is justified without additional information.

12.	Methanol can be synthesized by combining carbon monoxide and hydrogen. CO(g) + 2H2(g)   CH3OH(g); ΔH°rxn = -90.7 kJ A reaction vessel contains these compounds at equilibrium. What effect will be seen when equilibrium is re-established after decreasing the temperature by 45°C?

A.	All the partial pressures will decrease.

B.	The partial pressure of methanol will decrease.

C.	The partial pressures of hydrogen and methanol will decrease.

D.	The partial pressure of hydrogen will increase.

E.	The partial pressure of carbon monoxide will decrease.

13.	Which, if any, of the following acids is strong?

A.	phosphoric

B.

carbonic

C.

acetic

D.

water

E.	none of these choices is correct

14.	Which of the following acids has the lowest pH? 0.1 M HBO, pKa = 2.43 0.1 M HA, pKa = 4.55 0.1 M HMO, pKa = 8.23 0.1 M HST, pKa = 11.89 pure water

A.

HA

B.

HST

C.

HMO

D.

HBO

E.	pure water

15.	What is the pH of a 0.0125 M NaOH solution?

A.

0.972

B.

1.903

C.

12.097

D.

13.028

E.	none of these choices is correct

16.	What is the [OH^-] for a solution at 25°C that has [H3O^+] = 2.35 × 10^-3 M?

A.	4.26 × 10^-5 M

B.	2.35 × 10^-11 M

C.	4.26 × 10^-12 M

D.	2.35 × 10^-17 M

E.	none of these choices is correct

17.	What is the [H3O^+] for a solution at 25°C that has pOH = 5.640?

A.	2.34 × 10^-4 M

B.	2.29 × 10^-6 M

C.	4.37 × 10^-9 M

D.	4.27 × 10^-11 M

E.

8.360 M

18.	Select the pair of substances in which an acid is listed followed by its conjugate base.

A.

H^+, HCl

B.	NH3, NH4^+

C.	HPO4^2-, H2PO4^-

D.	HCO3^-, CO3^2-

E.	CH3COOH, CH3COOH2^+

19.	Butyric acid is responsible for the odor in rancid butter. A solution of 0.25 M butyric acid has a pH of 2.71. What is the Ka for the acid?

A.

0.36

B.	2.4 × 10^-2

C.	7.8 × 10^-3

D.	1.5 × 10^-5

E.	none of these choices is correct

20.	Formic acid, which is a component of insect venom, has a Ka = 1.8 × 10^-4. What is the [H3O^+] in a solution that is initially 0.10 M formic acid, HCOOH?

A.	4.2 × 10^-3 M

B.	8.4 × 10^-3 M

C.	1.8 × 10^-4 M

D.	1.8 × 10^-5 M

E.	1.8 × 10^-6 M

21.	What is the value of Kb for the cyanide anion, CN^-? Ka(HCN) = 6.2 × 10^-10

A.	1.6 × 10^-4

B.	1.6 × 10^-5

C.	3.8 × 10^-4

D.	3.8 × 10^-5

E.	6.2 × 10^4

22.	What is the pH of a 0.0100 M sodium benzoate solution? Kb (C7H5O2^-) = 1.5 × 10^-10

A.

0.38

B.

5.91

C.

8.09

D.

9.82

E.

13.62

23.	A solution is prepared by adding 0.10 mol of sodium fluoride, NaF, to 1.00 L of water. Which statement about the solution is correct?

A.	The solution is basic.

B.	The solution is neutral.

C.	The solution is acidic.

D.	The concentrations of fluoride ions and sodium ions will be identical.

E.	The concentration of fluoride ions will be greater than the concentration of sodium ions.

24.	A solution is prepared by adding 0.10 mol of iron(III) nitrate, Fe(NO3)3, to 1.00 L of water. Which statement about the solution is correct?

A.	The solution is basic.

B.	The solution is neutral.

C.	The solution is acidic.

D.	The value of Ka for the species in solution must be known before a prediction can be made.

E.	The value of Kb for the species in solution must be known before a prediction can be made.

25.	A solution is prepared by adding 0.10 mol of lithium nitrate, LiNO3, to 1.00 L of water. Which statement about the solution is correct?

A.	The solution is basic.

B.	The solution is neutral.

C.	The solution is weakly acidic.

D.	The solution is strongly acidic.

E.	The values for Ka and Kb for the species in solution must be known before a prediction can be made.

26.	An aqueous solution is prepared by dissolving the salt formed by the neutralization of a weak acid by a weak base. Which statement about the solution is correct?

A.	The solution is strongly basic.

B.	The solution is weakly basic.

C.	The solution is neutral.

D.	The solution is acidic.

E.	The values for Ka and Kb for the species in solution must be known before a prediction can be made.

27.	Which, if any, of the following aqueous mixtures would be a buffer system?

A.	CH3COOH, NaH2PO4

B.	H2CO3, HCO3^-

C.	H2PO4^-, HCO3^-

D.	HSO4^-, HSO3^-

E.	None of these will be a buffer solution.

28.	An acetate buffer has a pH of 4.40. Which of the following changes will cause the pH to decrease?

A.	dissolving a small amount of solid sodium acetate

B.	adding a small amount of dilute hydrochloric acid

C.	adding a small amount of dilute sodium hydroxide

D.	dissolving a small amount of solid sodium chloride

E.	diluting the buffer solution with water

29.	A buffer is prepared by adding 0.5 mol of solid sodium hydroxide to 1.0 L of 1.0 M acetic acid (CH3COOH). What is the pH of the buffer?

A.	The pH will be pKa - 0.30, where pKa is that of acetic acid.

B.	The pH will be greater than the pKa for acetic acid.

C.	The pH will be less than the value in answer a.

D.	The pH will be equal to the pKa for acetic acid.

E.	More information is needed to solve the problem.

30.	What is the pH of a buffer that consists of 0.45 M CH3COOH and 0.35 M CH3COONa? Ka = 1.8 × 10^-5

A.

4.49

B.

4.64

C.

4.85

D.

5.00

E.

5.52

31.	A formic acid buffer containing 0.50 M HCOOH and 0.50 M HCOONa has a pH of 3.77. What will the pH be after 0.010 mol of NaOH has been added to 100.0 mL of the buffer?

A.	3.67
B.	3.78
C.	3.81
D.	3.85
E.	3.95