To calculate the standard enthalpy of formation of Cl⁻(aq), we can use the provided thermochemical data and apply the principle of Hess’s Law, which states that if a reaction can be expressed as the sum of multiple reactions, the enthalpy change for the overall reaction is the sum of the enthalpy changes of the individual reactions.

Given reactions and their enthalpy changes:

1. $\text{HCl(g)} \to \text{H}^+(aq) + \text{Cl}^-(aq) \quad \Delta H = -75.15 \, \text{kJ}$
2. $\text{H}_2(g) + \text{Cl}_2(g) \to 2\text{HCl}(g) \quad \Delta H = -184.62 \, \text{kJ}$

Approach:

We will combine these reactions in such a way that the formation of Cl⁻(aq) appears as a product.

• Reaction 1 shows the dissociation of HCl(g) to form H⁺(aq) and Cl⁻(aq), but we need HCl(g) as a reactant.
• Reaction 2 provides HCl(g) from H₂(g) and Cl₂(g).

Now let's break down the strategy:

Step 1: Reverse Reaction 1

To get the formation of Cl⁻(aq) from HCl(g), we reverse Reaction 1:

[ \text{H}^+(aq) + \text{Cl}^-(aq) \to \text{HCl}(g) \quad \Delta H = +75.15 \