Question

Calculate pH for the following situations:

a) A solution containing 0.0357 M maleic acid and 0.051 M disodium maleate. The Ka values for maleic acid are 1.20 × 10-2 (Ka1) and 5.37 × 10-7 (Ka2).

b) A solution containing 0.0295 M succinic acid and 0.016 M potassium hydrogen succinate. The Ka values for succinic acid are 6.21 × 10-5 (Ka1) and 2.31 × 10-6 (Ka2).

Answer 1

(a) Maleic acid is related to K_a1 value whereas disodium maleate is related to K_a2 value. Hence, we have to use the average pK_a value to calculate the pH of solution.

K_a1 = 1.2 x 10^-2

pK_a1 = -log(K_a1)

pK_a1 = -log(1.2 x 10^-2)

pK_a1 = 1.92

K_a2 = 5.37 x 10^-7

pK_a2 = -log(K_a2)

pK_a2 = -log(5.37 x 10^-7)

pK_a2 = 6.27

Average pK_a = (pK_a1 + pK_a2) / 2

Average pK_a = (1.92 + 6.27) / 2

Average pK_a = 4.10

According to Henderson - Hasselbalch equation,

pH = pK_a + log([conjugate base] / [weak acid])

pH = pK_a + log([disodium maleate] / [maleic acid])

pH = 4.10 + log(0.051 M / 0.0357 M)

pH = 4.10 + log(1.43)

pH = 4.10+ 0.15

pH = 4.25

(b) Succinic acid and potassium hydrogen succinate are related to K_a1 value. Hence, we will use pK_a1 value to calculate pH

K_a1 = 6.21 x 10^-5

pK_a1 = -log(K_a1)

pK_a1 = -log(6.21 x 10^-5)

pK_a1 = 4.207

According to Henderson - Hasselbalch equation,

pH = pK_a + log([conjugate base] / [weak acid])

pH = pK_a1 + log([potassium hydrogen succinate] / [Succinic acid])

pH = 4.207 + log(0.016 M / 0.0295 M)

pH = 4.207 + log(0.54)

pH = 4.207 - 0.266

pH = 3.94