Question

The first and second dissociation constants of an acid H2A are 1.0 × 10–5 and 5.0 × 10–10 respectively. The overall dissociation constant of the acid will be

• A. 5.0 × 10–15
• B. 0.2 × 105
• C. 5.0 × 10–5
• D. 5.0 × 1015

Answer 1

Answer: (A)

5.0 × 10–15

Answer 2

H2A $\rightleftharpoons$ H+ + HA– ; K1 = $\frac{\lbrack H^{+}\rbrack\lbrack HA^{-}\rbrack}{\lbrack H_{2}A\rbrack}$ = 1 × 10–5

HA– $\rightleftharpoons$ H+ + A2– ;

K2 = 5 × 10–10 = $\frac{\lbrack H^{+}\rbrack\lbrack H^{2 -}\rbrack}{\lbrack HA^{-}\rbrack}$

K = $\frac{\lbrack H^{+}\rbrack^{2}\lbrack H^{2 -}\rbrack}{\lbrack H^{2}A^{-}\rbrack}$= K1 × K2 = 1 × 10–5 × 5 × 10–15