Question

buffer consists of 0.50 M NaH2PO4 and 0.40 M Na2HPO4. Phosphoric acid is a triprotic acid (K a 1 = 7.2 × 10 − 3, K a 2 = 6.3 × 10 − 8, and K a 3 = 4.2 × 10 − 13). (a) Which Ka value is most important to this buffer? (b) What is the buffer pH?

Answer 1

Answer:

The Ka value used for the buffer system is :- Ka2 = $6.3\times 10^{-8}$ pKa2 = 7.2

pH = 7.1

Explanation:

The buffer system given in the question is :-

$NaH_2PO_4$ and $Na_2HPO_4$

The reaction taking place is:-

$H_2PO_4^-\rightleftharpoons HPO_4^{2-}$

The Ka value used for the buffer system is :- Ka2 = $6.3\times 10^{-8}$ pKa2 = 7.2

The pH can be calculated as:-

$pH=pKa+\log\frac{Na_2HPO_4}{NaH_2PO_4}$

pH = 7.2 + log 0.40/0.50 = 7.1