Answer:
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
