Q.Assertion (A): The ionisation of hydrogen sulphide in water is low in the presence of hydrochloric acid.
Reason (R): Hydrogen sulphide is a weak acid.

The key concept here is the Common Ion Effect on the equilibrium of a weak acid.

1. Hydrogen sulphide ($H_2S$) is a weak acid, meaning its ionization in water is an equilibrium: $H_2S(aq) \rightleftharpoons H^+(aq) + HS^-(aq)$. This confirms Reason (R) is true.
2. Hydrochloric acid ($HCl$) is a strong acid that ionizes completely in water, producing a high concentration of $H^+$ ions: $HCl(aq) \rightarrow H^+(aq) + Cl^-(aq)$.
3. When $HCl$ is added to a solution of $H_2S$, the concentration of $H^+$ ions increases. According to Le Chatelier's principle, the $H_2S$ ionization equilibrium shifts to the left to consume the added $H^+$, thereby decreasing the ionization of $H_2S$. This confirms Assertion (A) is true. …

When an acid dissolves in water, it releases hydrogen ions ($H^+$). The extent to which it does this determines whether it is a strong or weak acid. Strong acids dissociate completely, while weak acids only partially dissociate, establishing an equilibrium. This equilibrium is sensitive to changes in concentration, as described by Le Chatelier's principle.

The core idea here is the common ion effect, which states that the solubility or ionization of a weak electrolyte is decreased by the addition of a strong electrolyte containing a common ion. For this effect to be significant, the initial electrolyte must be weak, as its equilibrium can be shifted.

Let's break down the assertion and reason:

1. Analyze Reason (R): Hydrogen sulphide is a weak acid.
   • Hydrogen sulphide ($H_2S$) is indeed a weak diprotic acid. This means it ionizes in two steps, but neither step proceeds to completion.
   • The first ionization equilibrium is:

*   The acid dissociation constant ($K_{a1}$) for this step is approximately $1.0 \times 10^{-7}$, which is a very small value. This small $K_a$ confirms that $H_2S$ dissociates only to a limited extent in water, making it a weak acid.
*   Therefore, Reason (R) is **true**.

2. Analyze Assertion (A): The ionisation of hydrogen sulphide in water is low in the presence of hydrochloric acid.

* Consider the ionization of $H_2S$ in water:

*   Now, consider what happens when hydrochloric acid ($HCl$) is added to this solution. Hydrochloric acid is a strong acid, meaning it dissociates completely in water:

*   The addition of $HCl$ significantly increases the concentration of $H^+$ ions in the solution.
*   According to Le Chatelier's principle, if a stress (like an increase in product concentration) is applied to a system at equilibrium, the system will shift in a direction that relieves that stress. In this case, the increased $H^+$ concentration from $HCl$ is a stress on the $H_2S$ equilibrium. …