Deck 14: Equilibria in Acid-Base Solutions

Question 1

An acetic acid-sodium acetate buffer is prepared by mixing 6.00 g NaCH₃CO₂ with 4.00 mL of CH₃CO₂H (density = 1.042 g/mL)and diluting to 1.00 L.What is the pH of the resulting buffer? The K_a of acetic acid is 1.8 × 10−⁵.

A) 4.10
B) 4.77
C) 4.99
D) 5.19
E) 5.78

Answer 1

4.77

Question 2

Which of the following equations is the Henderson-Hasselbalch equation?

A) $\mathrm{pH}=\mathrm{p} K_{\mathrm{a}}+\log \frac{\left[\mathrm{H}^{+}\right]}{\left[\mathrm{B}^{-}\right]}$
B) $\mathrm{pH}=\mathrm{p} K_{\mathrm{a}}-\log \frac{\left[\mathrm{B}^{-}\right]}{[\mathrm{HB}]}$
C) $\mathrm{pH}=\mathrm{p} K_{\mathrm{a}}+\log \frac{\left[\mathrm{B}^{-}\right]}{[\mathrm{HB}]}$
D) $\mathrm{p} K_{\mathrm{a}}=\mathrm{pH}+\log \frac{\left[\mathrm{B}^{-}\right]}{[\mathrm{HB}]}$
E) $\mathrm{p} K_{\mathrm{a}}=\mathrm{pH}-\log \frac{\left[\mathrm{OH}^{-}\right]}{\left[\mathrm{H}^{+}\right]}$

Answer 2

$\mathrm{pH}=\mathrm{p} K_{\mathrm{a}}+\log \frac{\left[\mathrm{B}^{-}\right]}{[\mathrm{HB}]}$

Question 3

A buffer is prepared by combining 25 mL of 0.50 M NH₃(aq)with 25 mL of 0.20 M HCl.What is the pH of the buffer? (K_a (NH₄⁺)= 5.6 × 10−¹⁰)

A) 7.76
B) 8.00
C) 8.86
D) 9.43
E) 9.65

Answer 3

9.43

Question 4

A buffer is prepared by combining 25 mL of 0.50 M NaF(aq)with 25 mL of 0.25 M HCl.What is the pH of the buffer? (K_a (HF)= 6.9 × 10−⁴)

A) 3.16
B) 3.50
C) 4.12
D) 4.60
E) 7.11

Question 5

What is the effect on pH when a buffer is diluted by a factor of 10?

A) The buffer pH deceases by 10.
B) The buffer pH decreases by 1.
C) The buffer pH decreases by 1..
D) The buffer pH increases by 10..
E) The buffer pH does not change appreciably.

Question 6

A buffer is prepared by combining 10.0 g of NaH₂PO₄ with 150 mL of 0.20 M NaOH and diluting to 2.0 L.What is the pH of the buffer? (K_a (H₂PO₄−)= 6.2 × 10−⁸)

A) 5.12
B) 6.76
C) 6.96
D) 7.20
E) 7.45

Question 7

What is the pH of the buffer that results when 12.0 g of NaH₂PO₄ and 8.00 g of Na₂HPO₄ are diluted with water to a volume of 0.50 L? (K_a of H₂PO₄^- = 6.2 × 10^-8,the molar masses of NaH₂PO₄ and Na₂HPO₄ are 120.0 g/mol and 142.0 mol,respectively)

A) 4.10
B) 6.96
C) 7.21
D) 7.46
E) 9.90

Question 8

The K_a of hypobromous acid,HOBr,is 2.6 × 10−⁹.Calculate the pH of a solution which is composed of 0.40 M HOBr and 0.40 M NaOBr.

A) 0.40
B) 0.80
C) 4.49
D) 8.59
E) 9.12

Question 9

What is the pH of the buffer that results when 32 g sodium acetate (NaCH₃CO₂)is mixed with 500.0 mL of 1.0 M acetic acid (CH₃CO₂H)and diluted with water to 1.0 L? (K_a of CH₃CO₂H = 1.8 × 10^-5)

A) 2.52
B) 4.23
C) 4.44
D) 4.64
E) 4.74

Question 10

The buffer capacity is

A) the amount of H⁺ or OH− that can be absorbed by a buffer without changing the pH appreciably.
B) the amount of buffer required to neutralize a strong acid or base.
C) a measure of the volume of buffer that a container may hold.
D) the volume of buffer required to neutralize 1.00 L of 1.00 M HCl.
E) dependent only on the ratio of base to acid in a buffer.

Question 11

The K_a of bicarbonate ion,HCO₃^-,is 4.8 × 10^-11.What [CO₃^2-]/[HCO₃^-] ratio is necessary to make a buffer with a pH of 11.00?

A) 0.21
B) 0.32
C) 0.68
D) 4.8
E) 6.8

Question 12

When mixed together,all of the following pairs can form buffers EXCEPT

A) H₃PO₄ and NaH₂PO₄.
B) NaH₂PO₄ and Na₂HPO₄.
C) CH₃CO₂H and NaOH.
D) HCl and NaCH₃CO₂.
E) NaI and NaOH.

Question 13

The K_a of acetic acid is 1.8 × 10−⁵.Calculate the pH of a solution that is composed of 1.0 M CH₃CO₂H and 0.50 M NaCH₃CO₂⁻.

A) 2.24
B) 4.44
C) 4.74
D) 5.05
E) 6.99

Question 14

What is the pH of a buffer that results when 0.50 mole of H₃PO₄ is mixed with 0.75 mole of NaOH and diluted with water to 1.00 L? (The acid dissociation constants of phosphoric acid are K_a1 = 7.1 × 10^-3,K_a2 = 6.2 × 10^-8,and K_a3 = 4.5 × 10^-13)

A) 1.82
B) 2.12
C) 6.91
D) 7.21
E) 12.44

Question 15

A buffer may be prepared by mixing a weak acid with a roughly equivalent amount of strong base.Which of the acids below is best for the preparation of a buffer with a pH of 9.00?

A) chlorous acid,HClO₂;K_a = 2.8 × 10−⁸
B) formic acid,HCO₂H;K_a = 1.9 × 10−⁴
C) benzoic acid,HC₇H₅O₂;K_a = 6.6 × 10−⁵
D) dihydrogen phosphate ion,H₂PO₄−;K_a = 6.2 × 10−⁸
E) ammonium ion,NH₄⁺;K_a = 5.6 × 10−¹⁰

Question 16

If the acid to base ratio in a buffer increases by a factor of 10,the pH of the buffer

A) decreases by 10.
B) decreases by 1.
C) increases by 1.
D) increases by 10.
E) is not affected.

Question 17

What is the pH of an aqueous solution composed of 0.64 M NH₄⁺ and 0.20 M NH₃? (K_a of NH₄⁺ = 5.6 × 10^-10)

A) 4.80
B) 8.75
C) 9.20
D) 9.25
E) 9.76

Question 18

What is the pH of a solution that results from diluting 0.30 mol acetic acid (CH₃CO₂H)and 0.20 mol sodium acetate (NaCH₃CO₂)with water to a volume of 1.0 L? (K_a of CH₃CO₂H = 1.8 × 10^-5)

A) 4.35
B) 4.57
C) 4.74
D) 4.92
E) 5.14

Question 19

A buffer may be prepared by mixing a weak acid with a roughly equivalent amount of strong base.Which of the acids below is best for the preparation of a buffer with a pH of 4.00?

A) sulfurous acid ,H₂SO₃;K_a = 1.7 × 10−²
B) hydrofluoric acid,HF;K_a = 6.9 × 10−⁴
C) benzoic acid,HC₇H₅O₂;K_a = 6.6 × 10−⁵
D) dihydrogen phosphate ion,H₂PO₄−;K_a = 6.2 × 10−⁸
E) ammonium ion,NH₄⁺;K_a = 5.6 × 10−¹⁰

Question 20

Which of the following pairs will form a buffer when mixed together in an aqueous solution?

A) KCl and KH₂PO₄
B) HCl and KOH
C) Ca(OH)₂ and NaOH
D) HF and NaF
E) None of the above will form a buffer.

Question 21

What volume of 0.2045 M NaOH is necessary to titrate 50.00 mL of 0.1177 M acetic acid?

A) 12.02 mL
B) 18.01 mL
C) 23.77 mL
D) 28.78 mL
E) 86.87 mL

Question 22

Which of the following solutions has the greatest buffer capacity?

A) A 1.0 L aqueous solution of 1.00 M HCl
B) A 1.0 L mixture of 1.00 M H₃PO₄ and 1.00 M H₂PO₄−
C) A 1.0 L mixture of 1.00 M H₃PO₄ and 0.500 M NaOH
D) A 1.0 L mixture of 1.00 M H₂PO₄− and 1.00 M HCl
E) A 1.0 L aqueous solution of 0.75 M H₂SO₄

Question 23

All of the following statements are false for the titration of a strong base by a strong acid EXCEPT

A) the equivalence point occurs at pH 7.
B) there will be at least two equivalence points,one for the base and one for the acid.
C) the equivalence point and the end point are identical,regardless of what indicator is used.
D) equal masses of acid and base are required to neutralize the solution.
E) phenolphthalein,if used as an indicator,will turn from colorless to pink.

Question 24

A volume of 25.0 mL of 0.100 M C₆H₅CO₂H(aq)is titrated with 0.100 M NaOH(aq).What is the pH after the addition of 12.5 mL of NaOH? (K_a of benzoic acid = 6.3 × 10^-5)

A) 2.60
B) 4.20
C) 5.40
D) 7.00
E) 8.60

Question 25

In a titration of HCl(aq)with NaOH(aq),the equivalence point is the point at which

A) equal volumes of HCl and NaOH have been combined.
B) the concentration of H⁺ equal 1.0 × 10^-14 M.
C) equal moles of HCl and NaOH have reacted.
D) half of the HCl has been neutralized by hydroxide ion.
E) the pH equals 14.00.

Question 26

If 25.00 mL of 0.2500 M formic acid is titrated with 25.00 mL of 0.2500 M NaOH,what is the pH at the equivalence point? The K_a for formic acid is 1.8 × 10−⁴.

A) 6.79
B) 8.42
C) 9.01
D) 9.67
E) 10.04

Question 27

Which one of the following conditions is always true for a titration of a weak acid with a strong base?

A) A colored indicator with a pK_a less than 7 should be used.
B) If a colored indicator is used,it must change color rapidly in the weak acid's buffer region.
C) Equal volumes of weak acid and strong base are required to reach the equivalence point.
D) The equivalence point occurs at a pH equal to 7.
E) The equivalence point occurs at a pH greater than 7.

Question 28

Which of the following conjugate acid-base pairs helps maintain blood at a pH of 7.40?

A) H₃O⁺ and H₂O
B) H₂PO₄− and HPO₄²⁻
C) H₃PO₄ and H₂PO₄−
D) H₂CO₃ and HCO₃−
E) HCl and Cl−

Question 29

Which of the following solutions has the least buffer capacity?

A) A 1.0 L mixture of 0.100 M H₃PO₄ and 0.100 M H₂PO₄−
B) A 1.0 L mixture of 0.200 M H₃PO₄ and 0.100 M NaOH
C) A 1.0 L mixture of 0.200 M H₂PO₄− and 0.100 M HCl
D) A 1.0 L mixture of 0.200 M H₂PO₄− and 0.100 M HNO₃
E) All of the above have the same buffer capacity.

Question 30

A buffer is prepared by adding 1.50 × 10² mL of 0.250 M NaOH to 2.50 × 10² mL of 0.350 M weak acid,HB.The solution is diluted to 1.00 L.If the pH of the resulting solution is 6.55,what is the pK_a of the weak acid?

A) 6.55
B) 6.67
C) 6.87
D) 6.92
E) 7.20

Question 31

The K_a for H₂PO₄− is 6.2 × 10−⁸.What volume of 0.10 M NaOH must be added to 0.50 L of 0.30 M H₂PO₄− to make a pH = 7.50 buffer?

A) 0.10 L
B) 0.13 L
C) 0.25 L
D) 0.51 L
E) 0.99 L

Question 32

A 25.0 mL sample of 0.400 M NH₃(aq)is titrated with 0.400 M HCl(aq).What is the pH at the equivalence point? (K_b of NH₃ = 1.8 × 10^-5)

A) 2.72
B) 4.97
C) 7.00
D) 9.03
E) 11.28

Question 33

If 25.00 mL of 0.200 M HCl is titrated with 0.200 M NaOH,what is the pH of the solution after the addition of 22.50 mL of NaOH?

A) 0.58
B) 1.98
C) 4.67
D) 6.88
E) 10.70

Question 34

A buffer contains 0.50 mol NH₄⁺ and 0.50 mol NH₃ diluted with water to 1.0 L.How many moles of NaOH are required to increase the pH of the buffer to 10.00? (pK_a of NH₄⁺ = 9.25)

A) 0.035 mol
B) 0.15 mol
C) 0.35 mol
D) 0.41 mol
E) 2.8 mol

Question 35

Sodium carbonate,Na₂CO₃,can be titrated with HCl according to the following balanced chemical equation. 2H⁺(aq)+ CO₃²⁻(aq)→ H₂O(l)+ CO₂(g)
What volume of 0.2000 M HCl is necessary to titrate 0.3462 g of sodium carbonate?

A) 16.33 mL
B) 21.74 mL
C) 32.67 mL
D) 37.79 mL
E) 41.11 mL

Question 36

A 30.00 mL sample of vinegar is titrated with 0.4190 M NaOH(aq).If the titration requires 27.83 mL of NaOH(aq),what is the concentration of acetic acid in the vinegar?

A) 0.1931 M
B) 0.2016 M
C) 0.2174 M
D) 0.3887 M
E) 0.4517 M

Question 37

All of the following solutions are buffers EXCEPT

A) 100 mL of 0.500 M H₂PO₄− added to 100 mL of 0.200 M HPO₄²⁻.
B) 100 mL of 0.200 M HF added to 200 mL of 0.200 M NaF.
C) 200 mL of 0.200 M HCl added to 200 mL of 0.400 M CH₃CO₂−.
D) 200 mL of 0.500 M NaOH added to 200 mL of 1.000 M HF.
E) 200 mL of 0.400 M HCl added to 100 mL of 0.200 M CH₃CO₂−.

Question 38

Potassium hydrogen phthalate (KHP)is used to standardize sodium hydroxide.If 35.39 mL of NaOH(aq)is required to titrate 0.8246 g KHP to the equivalence point,what is the concentration of the NaOH(aq)? (The molar mass of KHP = 204.2 g/mol) HC₈H₄O₄^-(aq)+ OH^-(aq) ? C₈H₄O₄^2-(aq)+ H₂O(l)

A) 0.02318 M
B) 0.05705 M
C) 0.0859 M
D) 0.1141 M
E) 0.1429 M

Question 39

What is the relationship between the pK_a for an acid-base indicator and the pH at which the indicator changes color?

A) pH = K_a
B) pH = pK_a
C) pH = −log[pK_a]
D) pH = −log[H⁺]
E) [H⁺] = pK_a

Question 40

A buffer is composed of 0.400 mol H₂PO₄^- and 0.400 mol HPO₄^2- diluted with water to a volume of 1.00 L.The pH of the buffer is 7.210.How many moles of HCl must be added to decrease the pH to 6.210?

A) 0.200 mol
B) 0.327 mol
C) 0.360 mol
D) 0.400 mol
E) 3.60 mol

Question 41

An impure sample of sodium carbonate,Na₂CO₃,is titrated with 0.113 M HCl according to the reaction below. 2 HCl(aq)+ Na₂CO₃(aq) ? CO₂(g)+ H₂O(l)+ 2 NaCl(aq)
What is the percent of Na₂CO₃ in a 0.613 g sample if the titration requires 26.14 mL of HCl? The molar mass of Na₂CO₃ is 106.0 g/mol.

A) 0.295%
B) 15.7%
C) 25.5%
D) 51.1%
E) 67.9%

Question 42

Natural rainfall is slightly acidic at a pH of about 5.5.Why is the pH of natural rain acidic?

A) pure water is acidic
B) HNO₃ forms when N₂ dissolves in rainwater
C) H₂CO₃ forms when CO₂ dissolves in rainwater
D) H₂O₃ forms when O₂ dissolves in rainwater
E) H₂SO₄ forms when smog dissolves in rainwater

Question 43

A 20.00-mL sample of 0.220 M triethylamine, (CH₃CH₂)₃N is titrated with 0.544 M HCl (K_b(CH₃CH₂)₃N = 5.2 × 10−⁴).What is the pH of the solution at the equivalence point?

A) 5.76
B) 8.24
C) 7.00
D) 3.28
E) 10.72

Question 44

In a titration experiment,if the initial solution pH is 4.0 and the equivalence point occurs at pH 9.0,then the reaction corresponds to

A) the titration of a weak acid by a strong base.
B) the titration of a weak acid by a weak base.
C) the titration of a weak base by a strong acid.
D) the titration of a strong acid by a strong base.
E) the titration of a strong base by a strong acid.

Question 45

Which two acids are responsible for acid rain?

A) HCl and HNO₃
B) HCl and H₂SO₄
C) HNO₃ and HF
D) HClO₄ and HNO₃
E) HNO₃ and H₂SO₄

Question 46

A 20.00-mL sample of 0.220 M triethylamine, (CH₃CH₂)₃N is titrated with 0.544 M HCl (K_b(CH₃CH₂)₃N = 5.2 × 10−⁴).How many mL of HCl are required to reach the equivalence point?

A) 16.17 mL
B) 21.74 mL
C) 32.67 mL
D) 8.09 mL
E) 49.45 mL

Question 47

Which of the following acids build up in muscles that are overexerted,causing pain?

A) Hydrochloric acid
B) Acetic acid
C) Carbonic acid
D) Hypochlorous acid
E) Lactic acid

Question 48

Which one of the following reactions describes the attack of acid rain on limestone?

A) 2H⁺(aq)+ Ca(s)→ Ca²⁺(aq)+ H₂(g)
B) 2H⁺(aq)+ Ca(OH)₂(s)→ Ca²⁺(aq)+ 2H₂O(l)
C) 2H⁺(aq)+ CaO(s)→ Ca²⁺(aq)+ H₂O(l)
D) 2H⁺(aq)+ CaCO₃(s)→ Ca²⁺(aq)+ CO₂(g)+ H₂O(l)
E) 3H⁺(aq)+ Al(OH)₃(s)→ Al³⁺(aq)+ 3H₂O(l)

Question 49

Which indicator is most appropriate for the titration of acetic acid with NaOH? The K_a for CH₃CO₂H is 1.8 × 10−⁵.

A) methyl red (pH 5)
B) bromthymol blue (pH 7)
C) phenolphthalein (pH 9)
D) Both methyl red and bromthymol blue are suitable.
E) All three indicators are suitable.