How do you calculate buffer pH for polyprotic acids?
All buffer solutions consist of a mixture of an acid with its conjugate base. Calculate pH using the Henderson-Hasselbalch equation, which requires the Ka of the acid and the relative concentrations of acid and base.
This works for polyprotic acids, too.
For a mixture of sodium carbonate and sodium bicarbonate, the bicarbonate is the acid and the carbonate is the base. Use Ka for bicarbonate and the relative concentrations as usual: