pH of 0.1 M NaHCO3 with given Ka values

[atavistic]

Homework Statement

Find pH of $$0.1M NaHCO_3$$

$$Ka_1 (H_2CO_3) = 4.3 * 10^{ - 7} , Ka_2 (H_2CO_3) = 5.61 * 10^{ - 11}$$

Homework Equations

$$pH = \frac {1}{2}(14 + pKa_1 + logC)$$

The Attempt at a Solution

Now $$K_h = \frac {Kw}{Ka_1} = 2.3 * 10^{ - 8} > > Ka2$$

So I neglect dissociation and use salt hydrolysis formula.

But this gives wrong answer.

 

[Borek]

pH of amphiprotic salt.