Question

A monobasic weak acid has a pka of 4.2 .The ph of the saturated solution of this acid is 3.1.Calculate the solubility of the acid

Answer 1

The solubility of this acid is 0.003185 M

Explanation:

A monobasic organic acid has a pK of 4.875. The pH of a saturated solution  of this acid is 3.00. Calculate the solubility of this organic acid in mol/dm3  units.

pH = 3.700, so [H+ ] = 10–pH = 10–3.7 = 1.9953 × 10-4 M

pK = 4.875, so Ka = 10–pK = 10–4.875 = 1.3335 × 10-5

[H+] = Ka  C(weak acid) - [H+] / [H+]

5

1.9953 x 10^-4 = 1.3335 x  10^-5  x C(weak acid) - 1.9953 x 10^-4/ 1.9953 x 10^-4

3.981 × 10-8 = 1.3335 × 10-5 (Cweak acid – 1.9953 ×10-4)

3.981 × 10-8 = 1.3335 × 10-5 Cweak acid – 2.661 ×10-9

4.247 × 10-8 = 1.3335 × 10-5 cweak acid

Cweak acid = 0.003185 M

Thus the solubility of this acid is 0.003185 M