Question

A solution contains 0.25 moles of non-volatile solute dissolved in one mole of solvent. Then calculate the % vapor pressure of solution relative to the vapor pressure of pure solvent.

• A. 20
• B. 40
• C. 60
• D. 80

Answer 1

The Correct Option is D

The vapor pressure of a solution is affected by the amount of solute present. Raoult's Law for the vapor pressure of a solution is given by:
\[ P_{\text{solution}} = P_{\text{solvent}} \times X_{\text{solvent}} \] Where:
- \( P_{\text{solution}} \) is the vapor pressure of the solution.
- \( P_{\text{solvent}} \) is the vapor pressure of the pure solvent.
- \( X_{\text{solvent}} \) is the mole fraction of the solvent in the solution.
The mole fraction of the solvent \( X_{\text{solvent}} \) is given by:
\[ X_{\text{solvent}} = \frac{n_{\text{solvent}}}{n_{\text{solvent}} + n_{\text{solute}}} \] Given:
- Moles of solute \( n_{\text{solute}} = 0.25 \)
- Moles of solvent \( n_{\text{solvent}} = 1 \)
Thus, the mole fraction of the solvent is:
\[ X_{\text{solvent}} = \frac{1}{1 + 0.25} = \frac{1}{1.25} = 0.8 \] The vapor pressure of the solution is then:
\[ P_{\text{solution}} = P_{\text{solvent}} \times 0.8 \] The percentage of vapor pressure of the solution relative to the vapor pressure of the pure solvent is:
\[ % \text{vapor pressure} = \frac{P_{\text{solution}}}{P_{\text{solvent}}} \times 100 = 0.8 \times 100 = 80% \] Step 1: Conclusion.
Thus, the % vapor pressure of the solution relative to the vapor pressure of pure solvent is \(80%\). Therefore, the correct answer is option (D).

Final Answer: 80