At a constant temperature, which of the following aqueous solutions will have the maximum vapour pressure?
(Mol wt `NaCl = 58.5, H_(2)SO_(4) = 98.0 g mol^(-1)`)

Options

A. 1 molal NaCl (aq)
B. 1 molar Nacl (aq)
C. 1 molal `H_(2)SO_(4)` (aq)
D. 1 molar `H_(2)SO_(4)` (aq)

Answer

To determine which aqueous solution has the maximum vapor pressure at constant temperature, we need to analyze the effect of solute concentration on vapor pressure.

### Step-by-Step Solution:

1. **Understanding Vapor Pressure**:
- Vapor pressure is the pressure exerted by a vapor in equilibrium with its liquid or solid phase at a given temperature. When a non-volatile solute is added to a solvent, the vapor pressure of the solvent decreases.

2. **Identifying the Solutions**:
- The options given are:
1. 1 M NaCl (sodium chloride)
2. 1 M H2SO4 (sulfuric acid)
3. 1 M NaCl (again)
4. 1 M H2SO4 (again)

3. **Determining the Number of Particles**:
- When dissolved in water, NaCl dissociates into 2 ions: Na⁺ and Cl⁻.
- H2SO4 dissociates into 3 ions: 2 H⁺ and SO4²⁻.
- Therefore, for 1 mole of each solute:
- 1 M NaCl → 2 particles (ions)
- 1 M H2SO4 → 3 particles (ions)

4. **Effect of Solute on Vapor Pressure**:
- The more particles present in solution, the lower the vapor pressure due to the colligative properties of solutions. This means that the vapor pressure decreases with an increase in the number of solute particles.
- Hence, the solution with fewer particles will have a higher vapor pressure.

5. **Comparing the Solutions**:
- For 1 M NaCl, we have 2 particles.
- For 1 M H2SO4, we have 3 particles.
- Since NaCl produces fewer particles than H2SO4, it will exert a higher vapor pressure.

6. **Conclusion**:
- The solution with the maximum vapor pressure is the one with 1 M NaCl.

### Final Answer:
The aqueous solution that will have the maximum vapor pressure at constant temperature is **1 M NaCl**.

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