Question

$K_{a1},K_{a2}$ and $K_{a3}$ are the respective ionisation constants for the following reactions;
$H_{2}S\rightleftharpoons H^{+}H{S}^{-}$
$H{S}^{-}\rightleftharpoons H^{+}{S}^{2-}$
$H_{2}S\rightleftharpoons 2H^{+}{S}^{2-}$
The correct relationship between $K_{a1},K_{a2}$ and $K_{a3}$ is:

• A. $K_{a3}=K_{a1}\times K_{a2}$
• B. $K_{a3}=K_{a1}+K_{a2}$
• C. $K_{a3}=K_{a1}-K_{a2}$
• D. $K_{a3}=K_{a1}/K_{a2}$

Answer 1

The correct option is A $K_{a3}=K_{a1}\times K_{a2}$

$H_{2}S\rightleftharpoons H^{+}+H{S}^{-}$ $K_{a1}$

$H{S}^{-}\rightleftharpoons H^{+}+S^{2-}$ $K_{a2}$

$H_{2}S\rightleftharpoons 2H^{+}+S^{2-}$ $K_{a3}$

$K_{a1}=\frac{\left[H{S}^{-}\right]\left[H^{+}\right]}{\left[H_{2}S\right]}$

$K_{a2}=\frac{\left[H^{+}\right]\left[S^{2-}\right]}{\left[H{S}^{-}\right]}$

$K_{a3}=K_{a1}.K_{a2}$