Deck 10: Monoprotic Acid-Base Equilibria

Question 1

Which of the following is NOT true for weak acids and weak bases?

A)K_w = K_a.K_b
B)
C)
D)pK_a = −log K_a
E)pK_b = −log K_b

Answer 1

Question 2

Calculate the percentage dissociation for a 0.010 M hypochlorous acid solution.The K_a for hypochlorous acid is 3.0 x 10⁻⁸.

A)0)35%
B)0)30%
C)0)17%
D)0)99%
E)0)65%

Answer 2

0)17%

Question 3

For a 0.0005 M HBr solution,the dissociation of water produces____________________ H₃O⁺ and____________________ OH⁻.

A)1)00E-07 M;2.00E-11 M
B)5)00E-04 M;5.00E-04 M
C)2)00E-11 M;2.00E-11 M
D)1)00E-07 M;1.00E-07 M
E)5)00E-04 M;2.00E-11 M

Answer 3

2)00E-11 M;2.00E-11 M

Question 4

Which is NOT a property of buffers?

A)Buffers resist changes in pH when acids or bases are added or when dilution occurs.
B)Buffers are a mixture of weak acid and conjugate base.
C)The pH of a buffer is independent of ionic strength.
D)The pH of a buffer is dependent on temperature.
E)Buffers are a mixture of weak bases and conjugate acid.

Question 5

The rule of thumb for neglecting the change in the formal concentration of a weak acid or a weak base,x,during an equilibrium problem is valid:

A)if the change in x is greater than 1% of the formal concentration.
B)if the change in x is less than 0.1% of the formal concentration.
C)if the change in x is equal to 0.1% of the formal concentration.
D)if the change in x is less than 10% of the formal concentration.
E)if the change in x is less than 1% of the formal concentration.

Question 6

Which of the following statements is NOT true for weak acids?

A)Weak acids are partially ionized.
B)The conjugate base is A⁻ and its strength is weak.
C)
D)
For the conjugate base.
E)HA + H₂O ⇋ A⁻ + H₃O⁺

Question 7

Calculate the pH for a 4.3 × 10⁻⁸ M HCl solution with an ionic strength of $\mu$ = 0.1. $\gamma$ _H+ = 0.83, $\gamma$ _OH− = 0.76 and $\gamma$ _Cl− = 0.755.

Question 8

There are seven common strong acids.Which of the following statements is NOT true for strong acids?

A)Dilute solutions are completely ionized.
B)HNO₂ is a strong acid.
C)For strong acid solutions with a concentration between 10⁻⁶ and 10⁻⁸ M,the pH is determined using systematic equilibrium.
D)For strong acid solutions with concentrations $\ge$ 10⁻⁶ M,the pH is calculated from the concentration of the strong acid.
E)For strong acid solutions with concentrations $\le$ 10⁻⁸ M,the pH is always 7.

Question 9

The pH of a weak-base solution is calculated using systematic treatment.Which of the following is NOT an equation needed to solve for pOH?

A)
B)F = [BH⁺] + [B]
C)[OH⁻] = [BH⁺] + [H⁺]
D)
E)K_w = [H⁺][OH⁻]

Question 10

A student calculates the pOH for a 0.01 M triethanolamine solution to be 4.12.To simplify the math she decided to neglect the change in the formal concentration.Was she justified in her decision? K_b = 5.78 × 10⁻⁷

A)She was justified,as the change in ×,7.59 × 10⁻⁵,is less than 1% of the formal concentration.
B)She was not justified,as the change in ×,9.92 × 10⁻³,is greater than 1% of the formal concentration.
C)She was justified,as the change in ×,1.32 × 10⁻¹⁰,is less than 1% of the formal concentration.
D)She was not justified,as the change in ×,1.00 × 10⁻²,is greater than 1% of the formal concentration.
E)She was justified,as the change in ×,5.78 × 10⁻⁵,is less than 1% of the formal concentration.

Question 11

A buffer is prepared by mixing 200.0 mL of 0.1500 M NaOH with 200.0 mL of 0.200 M CH₃CO₂H in a 1-L volumetric flask and then diluted to volume with distilled deionized water.Calculate the pH of the buffer.K_a = 1.75 × 10⁻⁵

A)5)23
B)9)72
C)7)00
D)8)77
E)4)28

Question 12

The pH of a weak-acid solution is calculated using systematic treatment.Which of the following is NOT an equation needed to solve for pH?

A)
B)F = [HA] + [A⁻]
C)[H⁺] = [A⁻] + [OH⁻]
D)
E)K_w = [H⁺][OH⁻]

Question 13

A 0.01 M solution of a weak base has a percent association of 18.32%.Calculate the K_b for the weak base.

A)1)83 × 10⁻³
B)2)43 × 10 ⁻¹¹
C)5)46 × 10⁻¹²
D)4)11 × 10⁻⁴
E)1)22 × 10⁻¹²

Question 14

Calculate the pH of 0.01 M HCN ( $\mu$ = 0.10)using activity coefficients,neglecting activity coefficients,and the percent difference between the two.K_a = 6.2 x 10⁻¹⁰ for HCN. $\gamma$ _H+ = 0.83, $\gamma$ _OH− = 0.76 and $\gamma$ _CN− = 0.755.

Question 15

Calculate the percent dissociation for a 0.01 M HCN solution,neglecting activity coefficients.K_a = 6.2 × 10⁻¹⁰ for HCN.

Question 16

There are two types of metal hydroxides,soluble metal hydroxides and insoluble hydroxides,both of which are strong bases.Which of the following statements is NOT true for strong bases?

A)Dilute solutions are completely dissociated.
B)Fe(OH)₂ is a strong soluble base.
C)For strong base solutions with a concentration between 10⁻⁶ and 10⁻⁸ M,the pOH is determined using systematic equilibrium.
D)For strong base solutions with concentrations $\ge$ 10⁻⁶ M,the pOH is calculated from the concentration of the strong base.
E)For strong base solutions with concentrations $\le$ 10⁻⁸ M,the pOH is always 7.

Question 17

The buffer needed for a capillary electrophoresis experiment must have a pH of 10.0.Which weak acid is the best choice for the buffer? Assume $\mu$ = 0.

A)ammonium chloride____________________ pKa = 9.24
B)disodium phosphate____________________ pK_a = 12.38
C)3-(Cyclohexylamino)propanesulfonic acid pK_a = 10.40
D)sodium dihydrogen borate____________________ pK_a = 12.74
E)cyclohexylaminoethanesulfonic acid____________________pK_a = 9.39

Question 18

Calculate the pH of a 0.2 M ethylamine solution.K_b = 5.0 × 10⁻⁴

Question 19

One liter of a pH 5.00 propionic acid buffer with a total concentration of 50 mM must be prepared.Calculate the mmols propionic acid and sodium propionate needed to prepare the buffer.K_a = 1.34 x 10^-5 for propionic acid.

Question 20

Which of the following statements is NOT true for weak bases?

A)Weak bases partially hydrolyze water.
B)The conjugate acid is BH⁺ and its strength is strong.
C)
D)
For the conjugate acid.
E)B + H₂O ⇋ BH⁺ + OH⁻