CHE 107 Spring 2018 Exam 3

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Question #: 1

Which two statements are true for the reaction N2O4(g) ⇔2 NO2(g) at equilibrium at 25 ºC? .

A. The concentrations of NO2 and N2O4 fluctuate about an average value.

B. At equilibrium, the reactions stop.

C. The rate of the reverse reaction [2 NO2(g) ⇔N2O4(g)] is twice the rate of the forward

reaction.

D. Kp for the reaction does not change when the pressure of NO2(g) is doubled.

E. At equilibrium, PN2O4 = (PNO2)2/KP

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Question #: 2

Which statement accurately describes the reaction? 2 NO2(g) ⇔ N2O4(g) KP = 63 at 0 ºC .

A. Reactants are favored.

B. Products are favored.

C. Neither reactants nor products are favored.

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Question #: 3

Calculate the equilibrium constant, KP at 298K, for 2 CH3OH(g) ⇔2 CO(g) + 4 H2(g) given CO(g) + 2 H2(g) ⇔CH3OH(g) KP = 2.26 × 104 at 298K.

A. 8.84 ×10–5

B. –4.52 ×104

C. 1.96 ×10–9

D. 4.42 ×10–19

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Question #: 4

In the synthesis of ammonia from nitrogen and hydrogen, the equilibrium constant Kc = 9.6 at 300.°C. N2(g) + 3 H2(g) ⇔ 2 NH3(g) What is the value of KP at this temperature? .

A. 9.6

B. 4.8

C. 4.8 × 102

D. 4.3 × 10–3

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Question #: 5

What is the Kc expression for 2 CaSO4(s) ⇔2 CaO(s) + 2 SO2(g) + O2(g)

A.

B. Kc = [SO2]2[O2]

C. Kc = [CaO]2[SO2]2[O2]

D.

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Question #: 6

Which two changes will increase the concentration of products? 2 CO(g) + 2 H2(g) ⇔ CH4(g) + CO2(g) ΔHrxn = –248 kJ

A. Increase volume.

B. Decrease volume.

C. Increase temperature.

D. Decrease temperature.

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Question #: 7

Which change will increase the concentration of H2O(g)? NiO(s) + H2(g) ⇔ Ni(s) + H2O(g) A. Addition of NiO(s).

B. Addition of H2(g).

C. Removal of Ni(s).

D. Removal of H2(g).

E. Removal of NiO(s).

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Question #: 8

At a certain temperature, the value of Kc for the reaction below is 8.5 ×10–3. If a reaction mixture at the same temperature contains 15.0 g of solid NH4HS, 0.015 M NH3 and 0.015 M H2S, what will occur? NH4HS(s) ⇔NH3(g) + H2S(g) .

A. The total pressure will decrease.

B. More of the solid will decompose.

C. The concentration of NH3 will decrease.

D. No changes will occur because the reaction is at equilibrium.

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Question #: 9

The compound, n-butane, can be converted to its isomer, isobutane. With both compounds in the gas phase, the equilibrium constant, Kp, is 2.5 at 25°C.

What is the equilibrium pressure of isobutane (in atm) in a container of the two isomers with a total pressure of 1.22 atm at 25°C? Report your answer with two significant figures. Do NOT include units in your answer. 1 atm

1.

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Question #: 10

The equilibrium constant, Kc, is 3.80 ×10–5 at 1000 K for I2(g) ⇔2 I(g). If one injects 0.0250 mol of I2(g) in a 100.0 mL reaction vessel, what is the equilibrium concentration of I(g) at 1000 K? 1 M Report your answer with three significant figures. Do NOT include units with your answer. Use the format 2.22E2 or 2.22E-2 for numbers in scientific notation.

1.

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Question #: 11

If the initial partial pressures of bromine and chlorine are both 0.20 atm, what is the equilibrium partial pressure of BrCl? 2 BrCl(g) ⇔ Br2(g)+ Cl2(g) KP = 9.0 × 103 at 150 K .

A. 9.5 × 10–5 atm

B. 4.0 × 10–3 atm

C. 8.3 × 10–4 atm

D. 2.1 × 10–3 atm

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Question #: 12

Which pair identifies a Brønsted-Lowry conjugate acid-base pair and the function of each substance in the reaction? ClO–(aq) + H2O(l) ⇔ HClO(aq) + OH–(aq) .

A. ClO–, base; OH–, conjugate acid

B. H2O, base; HClO, conjugate acid

C. ClO–, acid; HClO, conjugate base

D. H2O, acid; OH–, conjugate base

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Question #: 13

Given the data below, select the two true statements.

ºC Kw 100 5.13 × 10–13

50 5.48 × 10–14

0 1.14 × 10–15

A. The autoionization of water is endothermic.

B. The autoionization of water is exothermic.

C. In pure water, [H+] = 1.00 × 10–7 at 50°C

D. In pure water, [H+] = 2.34 × 10–7 at 50°C

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Question #: 14

A student measured the pH of an aqueous solution of 0.075 M sodium hydroxide at 50°C. Given that Kw = 5.48 × 10–14 at this temperature, what is the pH? .

A. 12.14

B. 12.88

C. 1.86

D. 1.12

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Question #: 15

Which two aqueous solutions are acidic at 25 °C?

A. pOH = 7.58

B. [H3O+] = 1.5 ×10–7 M

C. pH = 7.00

D. [OH–] = 6.5 ×10–7 M

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Question #: 16

Four 0.50 M aqueous monoprotic acid solutions (HA, HD, HX, HZ) were prepared and their pH values measured. Which one is the weakest acid?

A. HA, pH = 4.80

B. HD, pH = 4.63

C. HX, pH = 2.80

D. HZ, pH = 2.45

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Question #: 17

Which comparison of acidities is false?

A. H2SO4 is a stronger acid than H2SO3.

B. H2SeO3 is a stronger acid than H2TeO3.

C. HBrO3 is a stronger acid than HIO3.

D. HBrO2 is a stronger acid than HBrO3.

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Question #: 18

What is Ka of cyanic acid, HCNO, if a 0.100 M solution has a pH of 2.35? Ka = 1 Report your answer with two significant figures. Do NOT include units in your answer. Use the format 2.2E2 or 2.2E-2 for numbers in scientific notation.

1.

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Question #: 19

Uric acid, HC5H3N4O3, can accumulate in the joints and cause severe pain in patients with gout. What is the percent ionization of a 0.125 M uric acid solution? Ka(HC5H3N4O3) = 5.1 × 10–6

A. 8.0 ×10–4 %

B. 0.64 %

C. 1.3 %

D. 5.1 %

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Question #: 20

Determine the pH of a solution containing 0.075 M HClO4 and 0.500 M HClO (Ka = 3.47 × 10–8). Report your answer with two DECIMAL places. Do NOT include units in your answer. 1

1.

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Question #: 21

What is the pH of a 0.300 M solution of a weak base? Kb for the base is 8.4 ×10–6 at 25 °C. pH= 1 Report your answer with two DECIMAL places. Do NOT include units in your answer.

1.

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Question #: 22

A monoprotic acid, HX, has Ka = 5.6 ×10–9 at 25 °C. What is Kb for the conjugate base, X–, at the same temperature? Report your answer with two significant figures. Do NOT include units in your answer. Use the format 2.2E2 or 2.2E–2 for numbers in scientific notation. 1

1.

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Question #: 23

Which two 0.10 M aqueous salt solutions will have pH >7 at 25 °C?

A. NH4ClO4

B. LiNO3

C. RbC2H3O2

D. CH3CH2NH3Br

E. Na3PO4

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Question #: 24

A diprotic acid, H2A, has Ka1 = 3.8 ×10–4 and Ka2 = 2.4 ×10–8. What is the pH of a 0.400 M H2A aqueous solution? pH= 1 Report your answers with two DECIMAL places. Do NOT include units in your answer. Use the format 2.2E2 or 2.2E-2 for scientific notation.

1.

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Question #: 25

Buffers consist of significant amounts of either a __________ and its conjugate base OR a __________ and its conjugate acid.

A. strong acid, strong base

B. strong acid, weak base

C. weak acid, weak base

D. weak acid, strong base

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Question #: 26

Which combination will NOT act as a buffer?

A. 0.1 M H2CO3 and 0.4 M Na2CO3

B. 1.0 M C5H5NHCl and 1.5 M C5H5N

C. 0.50 M HNO2 and 1.5 M NaNO2

D. 0.020 M NaH2PO4 and 0.15 M K2HPO4

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Question #: 27

What is the pH of a buffer composed of 0.200 M acetic acid and 0.150 M sodium acetate? The pKa for acetic acid is 4.74. pH = 1 Report your answer with two DECIMAL places. Do NOT include units in your answer.

1.

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Question #: 28

What is the pH of a 1.00 L solution that is initially 0.500 M HC2H3O2 (pKa = 4.74) and 0.500 M NaC2H3O2, after the addition of 0.10 mol of HCl? Ignore any volume change from the addition of HCl. pH = 1

Report your answer to two DECIMAL places. Do NOT include units in your answer.

1.

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Question #: 29

Which conjugate acid-base pair is the most effective combination for preparing a buffer with pH = 6.75?

A. 0.100 M HClO,

0.100 M KClO,

pKa(HClO) = 7.54

B. 0.100 M HCN,

0.100 M NaCN,

pKa(HCN) = 9.31

C. 0.100 M C5H5NH+,

0.100 M C5H5N,

pKa(C5H5NH+) = 5.23

D. 0.100 M HC2H3O2,

0.100 M LiC2H3O2,

pKa(HC2H3O2) = 4.74

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Question #: 30

A chemistry student is asked to prepare a buffer solution containing benzoic acid (C6H5COOH;

pKa = 4.19) with a pH of 3.69. How many grams of sodium benzoate (NaC6H5COO, MW = 144

g/mol) would the student need to add to a 1.00 L solution of 0.4oo M C6H5COOH, assuming

no change in volume?

A. 0.126

B. 18.2

C. 57.6

D. 0.316

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CHE 107 Spring 2018 Exam 3 - Confidential

Your Name: Your ID:

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Question #: 1

Which two statements are true for the reaction N2O4(g) ⇔2 NO2(g) at equilibrium at 25 ºC?

.

A. The concentrations of NO2 and N2O4 fluctuate about an average value. B. At equilibrium, the reactions stop. C. The rate of the reverse reaction [2 NO2(g) ⇔N2O4(g)] is twice the rate of the forward

reaction. ✓D. Kp for the reaction does not change when the pressure of NO2(g) is doubled. ✓E. At equilibrium, PN2O4 = (PNO2)2/KP

Question #: 2

Which statement accurately describes the reaction?

2 NO2(g) ⇔ N2O4(g)    KP = 63 at 0 ºC

.

A. Reactants are favored.

✓B. Products are favored. C. Neither reactants nor products are favored.

Question #: 3

Calculate the equilibrium constant, KP at 298K, for 

   2 CH3OH(g) ⇔2 CO(g) + 4 H2(g)

given

    CO(g) + 2 H2(g) ⇔CH3OH(g)    KP = 2.26 × 104 at 298K.

A. 8.84 ×10–5 B. –4.52 ×104

✓C. 1.96 ×10–9

D. 4.42 ×10–19

Question #: 4

In the synthesis of ammonia from nitrogen and hydrogen, the equilibrium constant Kc = 9.6 at

300.°C.

N2(g) + 3 H2(g) ⇔ 2 NH3(g)

What is the value of KP at this temperature?

.

A. 9.6 B. 4.8 C. 4.8 × 102

✓D. 4.3 × 10–3

Question #: 5

What is the Kc expression for 2 CaSO4(s) ⇔2 CaO(s) + 2 SO2(g) + O2(g)?

.

A.  

✓B. Kc = [SO2]2[O2]

C. Kc = [CaO]2[SO2]2[O2]D.

Question #: 6

Which two changes will increase the concentration of products?

2 CO(g) + 2 H2(g) ⇔ CH4(g) + CO2(g)    ΔHrxn = –248 kJ

 

A. Increase volume.

✓B. Decrease volume. C. Increase temperature.

✓D. Decrease temperature.

Question #: 7

Which change will increase the concentration of H2O(g)?

NiO(s) + H2(g) ⇔ Ni(s) + H2O(g)    

. 

A. Addition of NiO(s).

✓B. Addition of H2(g). C. Removal of Ni(s).

D. Removal of H2(g). E. Removal of NiO(s).

Question #: 8

At a certain temperature, the value of Kc for the reaction below is 8.5 ×10–3.  If a reaction

mixture at the same temperature contains 15.0 g of solid NH4HS, 0.015 M NH3 and 0.015 M H2S, what will occur?

NH4HS(s) ⇔NH3(g) + H2S(g)

.

A. The total pressure will decrease.

✓B. More of the solid will decompose. C. The concentration of NH3 will decrease. D. No changes will occur because the reaction is at equilibrium.

Question #: 9

The compound, n-butane, can be converted to its isomer, isobutane.  With both compounds in

the gas phase, the equilibrium constant, Kp, is 2.5 at 25°C.

What is the equilibrium pressure of isobutane (in atm) in a container of the two isomers with a

total pressure of 1.22 atm at 25°C?

Report your answer with two significant figures. Do NOT include units in your answer.

    1     atm

1. 0.87|8.7e-1|.87|

Question #: 10

 The equilibrium constant, Kc, is 3.80 ×10–5 at 1000 K for I2(g) ⇔2 I(g). If one injects 0.0250

mol of I2(g) in a 100.0 mL reaction vessel, what is the equilibrium concentration of I(g) at

1000 K?

    1     M

Report your answer with three significant figures. Do NOT include units with your answer. Use

the format 2.22E2 or 2.22E-2 for numbers in scientific notation. 

1. 3.08E-3|0.00308|.00308|3.08 E-3|

Question #: 11

If the initial partial pressures of bromine and chlorine are both 0.20 atm, what is the equilibrium

partial pressure of BrCl?

2 BrCl(g)  ⇔  Br2(g)+ Cl2(g)    KP = 9.0 × 103 at 150 K .

A. 9.5 × 10–5 atm B. 4.0 × 10–3 atm C. 8.3 × 10–4 atm

✓D. 2.1 × 10–3 atm

Question #: 12

Which pair identifies a Brønsted-Lowry conjugate acid-base pair and the function of each

substance in the reaction?

ClO–(aq) + H2O(l) ⇔ HClO(aq) + OH–(aq)

.

A. ClO–, base;  OH–, conjugate acid B. H2O, base; HClO, conjugate acid C. ClO–, acid;  HClO, conjugate base

✓D. H2O, acid; OH–, conjugate base

Question #: 13

Given the data below, select the two true statements.

 

✓A. The autoionization of water is endothermic.

B. The autoionization of water is exothermic. C. In pure water, [H+] = 1.00 × 10–7 at 50°C

✓D. In pure water, [H+] = 2.34 × 10–7 at 50°C

Question #: 14

A student measured the pH of an aqueous solution of 0.075 M sodium hydroxide at 50°C.

Given that Kw = 5.48 × 10–14 at this temperature, what is the pH? 

.

✓A. 12.14

B. 12.88 C. 1.86 D. 1.12

Question #: 15

Which two aqueous solutions are acidic at 25 °C?

✓A. pOH = 7.58 ✓B. [H3O+] = 1.5 ×10–7 M

C. pH = 7.00 D. [OH–] = 6.5 ×10–7 M

Question #: 16

ºC Kw100 5.13 × 10–13

50 5.48 × 10–14

0 1.14 × 10–15

Four 0.50 M aqueous monoprotic acid solutions (HA, HD, HX, HZ) were prepared and their pH

values measured. Which one is the weakest acid?

✓A. HA, pH = 4.80

B. HD, pH = 4.63 C. HX, pH = 2.80 D. HZ, pH = 2.45

Question #: 17

Which comparison of acidities is false?

A. H2SO4 is a stronger acid than H2SO3. B. H2SeO3 is a stronger acid than H2TeO3. C. HBrO3 is a stronger acid than HIO3.

✓D. HBrO2 is a stronger acid than HBrO3.

Question #: 18

What is Ka of cyanic acid, HCNO, if a 0.100 M solution has a pH of 2.35?

Ka =     1    

 

Report your answer with two significant figures. Do NOT include units in your answer. Use the

format 2.2E2 or 2.2E-2 for numbers in scientific notation.

1. 2.1E-4|2.1 E-4|2.0E-4|

Question #: 19

Uric acid, HC5H3N4O3, can accumulate in the joints and cause severe pain in patients with

gout. What is the percent ionization of a 0.125 M uric acid solution?

Ka(HC5H3N4O3) = 5.1 × 10–6 

.

A. 8.0 ×10–4 % ✓B. 0.64 %

C. 1.3 % D. 5.1 %

Question #: 20

Determine the pH of a solution containing 0.075 M HClO4 and 0.500 M HClO (Ka = 3.47 × 10–

8).

Report your answer with two DECIMAL places. Do NOT include units in your answer.

    1    

1. 1.12

Question #: 21

What is the pH of a 0.300 M solution of a weak base? Kb for the base is 8.4 ×10–6 at 25 °C.

pH=     1    

Report your answer with two DECIMAL places. Do NOT include units in your answer.

1. 11.20

Question #: 22

A monoprotic acid, HX, has Ka

= 5.6 ×10–9 at 25 °C. What is Kb

 for the conjugate base, X–, at

the same temperature?

Report your answer with two significant figures. Do NOT include units in your answer. Use the

format 2.2E2 or 2.2E–2 for numbers in scientific notation.

    1    

1. 1.8E-6|0.0000018|1.8 E-6|

Question #: 23

Which two 0.10 M aqueous salt solutions will have pH >7 at 25 °C? 

A. NH4ClO4 B. LiNO3

✓C. RbC2H3O2 D. CH3CH2NH3Br

✓E. Na3PO

4

Question #: 24

A diprotic acid, H2A, has Ka1 = 3.8 ×10–4 and Ka2 = 2.4 ×10–8.  What is the pH of a 0.400 M H

2A aqueous solution?

pH=     1        

Report your answers with two DECIMAL places. Do NOT include units in your answer. Use the

format 2.2E2 or 2.2E-2 for scientific notation.

1. 1.91

Question #: 25

Buffers consist of significant amounts of either a __________ and its conjugate base OR

a __________ and its conjugate acid. 

A. strong acid, strong base B. strong acid, weak base

✓C. weak acid, weak base D. weak acid, strong base

Question #: 26

Which combination will NOT act as a buffer?

✓A. 0.1 M H2CO3 and 0.4 M Na2CO3

B. 1.0 M C5H5NHCl and 1.5 M C5H5N C. 0.50 M HNO2 and 1.5 M NaNO2 D. 0.020 M NaH2PO4 and 0.15 M K2HPO4

Question #: 27

What is the pH of a buffer composed of 0.200 M acetic acid and 0.150 M sodium acetate? The p

Ka for acetic acid is 4.74.

pH =     1    

Report your answer with two DECIMAL places. Do NOT include units in your answer.

1. 4.62

Question #: 28

What is the pH of a 1.00 L solution that is initially 0.500 M HC2H3O2 (pKa = 4.74) and

0.500 M NaC2H3O2, after the addition of 0.10 mol of HCl? 

Ignore any volume change from the addition of HCl.

pH =     1    

Report your answer to two DECIMAL places. Do NOT include unitsin your answer.

1. 4.56

Question #: 29

Which conjugate acid-base pair is the most effective combination for preparing a buffer with pH

= 6.75?

✓A. 0.100 M HClO,

0.100 M KClO, pKa(HClO) = 7.54

B. 0.100 M HCN, 0.100 M NaCN, pKa(HCN) = 9.31

C. 0.100 M C5H5NH+, 0.100 M C5H5N, pKa(C5H5NH+) = 5.23

D. 0.100 M HC2H3O2, 0.100 M LiC2H3O2, pKa(HC2H3O2) = 4.74

Question #: 30

A chemistry student is asked to prepare a buffer solution containing benzoic acid (C6H5COOH;

pKa = 4.19) with a pH of 3.69. How many grams of sodium benzoate (NaC6H5COO, MW =

144 g/mol) would the student need to add to a 1.00 L solution of 0.4oo M C6H5COOH,

assuming no change in volume?

 

A. 0.126

✓B. 18.2 C. 57.6 D. 0.316