Calculate the pH of a 0.020 M H2CO3 solution. At 25 °C, Ka1 = 4.3 * 10-7. H2CO3 (aq) + H2O (l) ↔ H3O + (aq) + HCO3 - (aq)

H₂CO₃ (aq) + H₂O (l) ↔ H₃O⁺ (aq) + HCO₃⁻ (aq)

Let d be the degree of dissociation

.02 (1-d).02d. 02d

Dissociation constant Ka₁ is given

4.3 x 10⁻⁷ =.02d x. 02d /.02 (1-d)

=.004 d² /.02 (neglecting d in denominator)

=.02 d²

d² = 215 x 10⁻⁷

d = 4.636 x 10⁻³

=.004636

concentration of H₃O⁺

= d x. 02

=.004636 x. 02

= 9.272 x 10⁻⁵

pH = - log [ H₃O⁺ ]

= - log (9.272 x 10⁻⁵)

5 - log 9.272

= 5 -.967

= 4.033.