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 decreases

Answer 1

Hint: An acidic species releases hydrogen ions. Thus when an acidic species is added to an acidic species it will show a common ion effect due to the presence of the same hydrogen ion. Thus the concentration of the corresponding ions decreases.

Complete Step by Step Answer:
Hydrogen sulphide is a weak acid. It dissociates in solution as hydrogen ion and sulphide ion. The dissociation is given as follows-
\[{{H}_{2}}S\rightleftharpoons {{H}^{+}}+H{{S}^{-}}\]
Hydrochloric acid is a strong acid. Thus it dissociates in hydrogen ions and chloride ions. The dissociation is given as follows-
$HCl\rightleftharpoons {{H}^{+}}+C{{l}^{-}}$
Thus in the reaction of hydrogen sulphide when hydrochloric acid is added the concentration of the hydrogen ion increases and it will show a common ion effect. Thus to maintain the value of the solubility product the concentration of the $H{{S}^{-}}$ion decreases.
Thus the correct option is D.

Note: Due to common ion effect the solubility of a species in a solution decreases and it can precipitate easily. Here hydrogen ion acts as a common ion as it is present in both hydrogen sulphide and hydrochloric acid.