If it is known that `H_2S` is a weak acid that ionizes to form `2H^+` and `S^(2-)` , lowering the pH of a solution of `H_2S` by adding HCl would

Options

A. lower the `S^(2-)` concentration
B. have no effect on `S^(2-)` concentration
C. raise the `S^(2-)` concentration
D. not be possible.

Explanation

To solve the problem, we need to analyze the effect of adding HCl to a solution of H2S, a weak acid that ionizes to form H+ ions and S2- ions. Let's break down the solution step by step.

### Step-by-Step Solution:

1. **Understanding the Ionization of H2S:**
H2S is a weak acid that ionizes in water according to the following reaction:
\[ \text{H}_2\text{S} \rightleftharpoons 2\text{H}^+ + \text{S}^{2-} \]
This means that for every molecule of H2S that ionizes, it produces 2 H+ ions and 1 S2- ion.

**Hint:** Remember that weak acids do not completely dissociate in solution, and their ionization is reversible.

2. **Effect of Adding HCl:**
When HCl, a strong acid, is added to the solution, it dissociates completely to produce additional H+ ions:
\[ \text{HCl} \rightarrow \text{H}^+ + \text{Cl}^- \]
This increases the concentration of H+ ions in the solution.

**Hint:** Strong acids fully dissociate in solution, which means they contribute significantly to the H+ concentration.

3. **Impact on pH:**
The addition of HCl lowers the pH of the solution. According to the pH scale, a lower pH indicates a higher concentration of H+ ions.

**Hint:** Recall that pH is inversely related to the concentration of H+ ions: lower pH = higher H+ concentration.

4. **Common Ion Effect:**
The increase in H+ concentration due to the addition of HCl will affect the equilibrium of the ionization of H2S. According to Le Chatelier's principle, if the concentration of one of the products (H+) increases, the equilibrium will shift to the left to counteract this change.

This means that the reaction will favor the formation of H2S, resulting in a decrease in the concentration of S2- ions:
\[ \text{H}_2\text{S} \leftarrow 2\text{H}^+ + \text{S}^{2-} \]

**Hint:** Le Chatelier's principle states that a system at equilibrium will adjust to minimize the effect of a change in concentration, temperature, or pressure.

5. **Conclusion:**
Therefore, by adding HCl and lowering the pH, the concentration of S2- ions in the solution will decrease.

**Final Answer:** The addition of HCl will lower the concentration of S2- ions in the solution of H2S.