What is the ratio of the concentrations of H2PO4 and HPO4 2 at (a) pH 7.0; (b) pH 7.5; (c) pH 8.0?

A. 1.62

B. 0.513

C. 0.162.

Explanation:

pH = pKa + log [ HPO42-] / [H2PO4-]

Assuming, pKa is 7.21

Ka = 6.2 x 10^-8

pKa = 7.21

A.

7.0 = 7.21 + log [HPO42-] / [HPO4-]

- 0.21 = log [ HPO42-] / [H2PO4-]

[ HPO42-] / [ H2PO4-] = 10^-0.21

= 0.617

[H2PO4-] / [HPO42-] = 1 / 0.617

= 1.62

B.

7.5 = 7.21 + log [HPO42-] / [H2PO4-]

0.29 = log [HPO42-] / [H2PO4-]

[HPO42-] / [H2PO4-] = 10^0.29

= 1.95

[H2PO4-] / [HPO42-] = 1/1.95

= 0.513

8.0 = 7.21 + log [HPO42-] / [ H2PO4-]

0.79 = log [HPO42-] / [H2PO4-]

[HPO42-] / [H2PO4-] = 10^0.79

= 6.17

[H2PO4-] / [HPO42-] = 1/6.17

= 0.162