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

Answer: (D)

$K_a$ for $H_{2}S=\frac{[H^{+}][H{S}^{-}]}{[H_{2}S]}$

Due to common ion effect an increase in $[H^{+}]$ will show a decrease in $[H{S}^{-}]$ as dissociation of $H_{2}S$ will decrease.