Question

1. (3 points) A 0.0730 M solution of a monoprotic acid is 1.07% ionized.

1. What is the pH of the solution?

2. Calculate the K_a of the acid.

2. (2 points) The pH of a 0.025 M solution of a monoprotic acid is 3.21. What is the Ka value for the acid?

3. (2 points) Determine the percent ionization of a 0.0028 M HA solution. (K_a of HA = 1.4 x 10^-9)

4. (4 points) Lysine is triprotic amino acid (separately loses three H’s in solution). The pKa’s are 2.18, 8.95, and 10.53.

1. Write the three ionization equilibrium reactions for lysine (C₆H₁₅N₂O₂).

2. Determine the concentration of each species in a 0.10 M solution.

5. (8 points) Calculate the pH of each of the following aqueous solutions at 25 °C.

1. 0.65 M boric acid (B(OH)₃, K_a = 7.3 x 10^-10)

2. 3.15 M ammonia (NH₃, K_b = 1.76 x 10^-5)

3. 0.82 M benzoic acid (C₆H₅COOH, K_a = 6.3 x 10^-5)

4. 0.100 M H₃AsO₄ (K_a1 = 2.5 x 10^-4, K_a2 = 5.6 x 10^-8, K_a3 = 3.0 x 10^-13)

6. (2 points) Determine whether each of the following salts are acidic, basic, or neutral.

1. Na₂SO₄

2. CH₃NH₃OI           (K_b of CH₃NH₂ = 4.4 x 10^-4; K_a of HOI = 2.3 x 10^-11)

3. KOBr

4. C₅H₅NHNO₃

7. (4 points) LiCN is a salt solution. The Ka of HCN is 6.2 x 10^-10.

1. Should the salt solution be acidic, basic, or neutral?

2. Write the equilibrium reaction for the ion of the salt that contributes to the pH of the solution.

3. Calculate the pH of a 1.25 M solution of LiCN.

Answer 1

a) The concentration of released hydronium is calculated:

[H3O +] = 0.0107 * 0.073 M = 0.00078 M

The pH is calculated:

pH = - log 0.00078 = 3.11

b) The Ka is calculated:

Ka = [H3O +] * [A-] / [HA] = (0.00078) ^ 2 / 0.073 - 0.00078 = 8.42x10 ^ -6

c) The hydronium concentration is calculated:

[H3O +] = 10 ^ -pH = 10 ^ -3.21 = 6.17x10 ^ -4 M

The Ka is calculated:

Ka = (6.17x10 ^ -4) ^ 2 / 0.025 - 6.17x10 ^ -4 = 1.56x10 ^ -5

d) The hydronium concentration is calculated from the expression of Ka:

1.4x10 ^ -9 = X ^ 2 / 0.0028

It clears X = 1.98x10 ^ -6 M

The ionization percentage is calculated:

% i = X * 100 / [HA] = 1.98x10 ^ ⁻6 * 100 / 0.0028 = 0.07%

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