Deck 11: Polyprotic Acid-Base Equilibria

Question 1

Calculate the pH of a 0.200 M H₂A⁻ solution.K₁ = 1 × 10⁻⁴,K₂ = 1 × 10⁻⁸ and K₃ = 1 × 10⁻¹².

A)4)35
B)6)00
C)8)00
D)8)65
E)10.00

Answer 1

10.00

Question 2

Solid NaOH is added to 1 L of an isoleucine solution,H₂L⁺,i.e,100 mM.When 175 mmol of NaOH is added,the principal species is____________________ .The pKa for the carboxylic acid group is 2.318 and the pKa for the ammonium group is 9.758.Assume the isoleucine solution volume does not change appreciably.

A)H₂L⁺
B)L⁻
C)HL
D)H₃L²⁺
E)L²⁻

Answer 2

L⁻

Question 3

Calculate the concentration of each species of oxalic acid in a 0.150 M solution of sodium oxalate,Na₂C₂O₄.K₁ = 5.62 × 10⁻²,K₂ = 7.24 × 10⁻⁵

Answer 3

[A²⁻] = 0.150 M,[HA⁻] = 4.55 × 10⁻⁶ M,[H₂A] = 1.78 × 10⁻¹³

Question 4

Calculate the pH for a 0.200 M sodium L−aspartic acid solution in the H₂L form.pK_a1 = 1.990,pK_a2 = 3.900 and pK_a3 = 10.002.

Question 5

Malonic acid is a diprotic acid with pKs of 2.847 and 5.696.Calculate the $\alpha$ for each malonic acid species in solution at pH =3.50.

Question 6

The fractional composition diagram below is representative for diprotic weak acids.Which point INCORRECTLY indicates the principal species?

A)H₂A
B)HA⁻
C)A⁻
D)H₂A = HA⁻
E)HA⁻ = A²⁻

Question 7

Alanine is a diprotic amino acid with a pK_a = 2.344 for the carboxylic group and a pK_a = 9.868 for the ammonium group.At pH = 6.32,what is the principal species in solution?

A)H₂L⁺
B)L⁻
C)HL
D)H₃L²⁺
E)L²⁻

Question 8

Which of the answers below is INCORRECT for diprotic acids and bases?

A)Solutions of H₂A are treated as monoprotic to calculate [H⁺],[HA⁻] and [H₂A].
B)The pH for HA⁻ solutions is appro×imated with pH = ½(pK₁ + pK₂).
C)K_a1K_b1 = K_w and K_a2K_b2 = K_w
D)Solutions of A²⁻ is treated as monobasic to calculate [OH⁻],[HA⁻],and [A²⁻].
E)A zwitterion is a neutral molecule with both positive and negative charges.

Question 9

Calculate the pH of a solution prepared by mixing 200.0 mL of 0.100 M NaOH with 150.0 mL of 0.200 M o×alic acid.K₁ = 5.62 × 10⁻²,K₂ = 7.24 × 10⁻⁵

Question 10

Which of the following is TRUE for an aqueous solution of the weak acid H₃A?
I H₃A is acidic
II H₂A⁻ is acidic
III HA²⁻ is amphiprotic
IV A³⁻ is basic

A)II,III,and IV
B)I,II,and IV
C)I,II,and III
D)I,III,and IV
E)I,II,III,and IV

Question 11

Calculate the pH of a 0.340 M NaO₂CCO₂H solution.pK₁ = 1.250 and pK₂ = 4.266.

A)2)37
B)4)89
C)2)79
D)3)38
E)−0.029

Question 12

For the polyprotic acid H₃A,which of the equilibria below is INCORRECT?

A)A³⁻ ⇋ HA²⁻ + OH⁻ K_b3 = K_w/K_a3
B)H₂A⁻ ⇋ HA²⁻ + H⁺ K_a2 = K₂
C)HA²⁻ ⇋ A³⁻ + H⁺ K_a3 = K₃
D)H₃A ⇋ H₂A⁻ + H⁺ K_a1 = K₁
E)HA²⁻ ⇋ H₂A⁻ + OH⁻ K_b2 = K_w/K_a2

Question 13

Sulfurous acid is a diprotic acid with K_a1 = 1.40 × 10⁻² and K_a2 = 6.73 × 10⁻⁸.Which of the statements below are FALSE?
I K_b2 = 7.14 × 10⁻¹³
II K_b1 = 1.49 × 10⁻⁷
III For 0.1 M H₂SO₃,[H⁺] = 7.61 × 10⁻² for K_a1.
IV For 0.1 M NaHSO₃,pH $\approx$ 4.515.

A)I,II,and IV
B)I,II,and III
C)I and II
D)I,II,III,and IV
E)II,III,and IV

Question 14

Sodium hydroxide is added to a 0.1 M solution of phenylalanine.Phenylalanine is a diprotic acid with two pK_as,2.20 for the carboxylic acid group and 9.31 for the ammonium functional group.When the pH = 5.93,the principal species is____________________ and when the pH = 10.10,the principal species is____________________ .

A)H₂L⁺;H₂L⁺
B)HL;L⁻
C)H₂L⁺;L⁻
D)H₂L⁺;HL
E)HL;HL

Question 15

Calculate the pH at the isoelectric point for a 0.10 M solution of the amino acid threonine.pK_a1 = 2.088 and pK_a2 = 9.100 for threonine.

A)3)57
B)2)39
C)5)59
D)1)54
E)5)05

Question 16

Which is NOT true for isoelectric and isoionic pH?

A)Isoionic pH is the pH of the pure,neutral,polyprotic acid.
B)Isoelectric pH is the pH at which average charge of the polyprotic acid is +1.
C)The hydronium concentration at the isoionic point is calculated with the equation

)
D)The pH at the isoelectric point is calculated with the equation pH = ½(pK₁ + pK₂).
E)For the equation

,F is the formal concentration of the weak acid.

Question 17

A buffer is prepared by dissolving 70 millimoles of solid NaOH in 500.0 mL of solution with a concentration of 50 mM H₂A.Which of the following are NOT true for the buffer?
I After reaction there is 30 mmol HA⁻ and 20 mmol A²⁻ in solution.
II pH is calculated using .
III After reaction there is 50 mmol HA⁻ and 20 mmol NaOH in solution.
IV pH is calculated using .
V pH = 14 - pOH where pOH is calculated from excess OH⁻.

A)III,IV,and V
B)II,III,IV,and V
C)1,III,and IV
D)I,II,and V
E)I and II

Question 18

For a weak acid,the $\alpha$ _HA____________________ and the $\alpha$ _A-____________________ with increased pH.

A)increases;decreases
B)increases;increases
C)decreases;decreases
D)decreases;increases
E)is fixed ;is fixed

Question 19

Calculate the pH to which a malonic acid solution must be adjusted to reach the isoelectric point.pK₁ = 2.847 and pK₂ = 5.696.

Question 20

How many millimoles of NaOH or HCl must be added to 500.0 mL of 0.250 M NaO₂CCO₂H to prepare a pH 3.90 buffer? Assume no change in volume.pK₁ = 1.250 and pK₂ = 4.266.

A)37.65 mmol NaOH
B)87.35 mmol HCl
C)124.7 mmol HCl
D)0)30 mmol NaOH
E)The answer cannot be calculated from given information.