Question

An aqueous solution of hydrogen sulphide shows the equilibrium,
$H_{2}S\rightleftharpoons H^{+}+H{S}^{-}$
If dilute hydrochloric acid is added to an aqueous solution of hydrogen sulphide without any change in temperature, then :

• A. the equilibrium constant will change
• B. the concentration of $H{S}^{-}$ will increase
• C. the concentration of undissociated hydrogen sulphide will decrease
• D. the concentration of $H{S}^{-}$ will decrease

Answer 1

The correct option is D the concentration of $H{S}^{-}$ will decrease

Reaction:
$H_{2}S\rightleftharpoons H^{+}+H{S}^{-}$

As we know, for this reaction
$K_a$ for $H_{2}S=\frac{\left[H^{+}\right]\left[H{S}^{-}\right]}{\left[H_{2}S\right]}$

An increase in $\left[H^{+}\right]$ will show a decrease in $\left[H{S}^{-}\right]$ to maintain constant $K_a$ value.