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Vapour pressure of chloroform (CHCl3) and dichloromethane (CH2Cl2) at 25 °C are 200 mm Hg and 415 mm Hg respectively. Vapour pressure of the solution obtained by mixing 25.5 g of CHCl3 and 40 g of CH2Cl2 at the same temperature will be: (Molecular mass of CHCl3 = 119.5 u and molecular mass of CH2Cl2 = 85 u).

1. 173.9 mm Hg
2. 615.0 mm Hg
3. 347.9 mm Hg
4. 285.5 mm Hg

Correct answer: 285.5 mm Hg

Solution

To calculate the vapour pressure of the solution, we use Raoult's Law. First, calculate the moles of CHCl3 (25.5 g / 119.5 g/mol = 0.213 mol) and CH2Cl2 (40 g / 85 g/mol = 0.471 mol). Then, find the mole fractions: X_CHCl3 = 0.213 / (0.213 + 0.471) = 0.311 and X_CH2Cl2 = 0.471 / (0.213 + 0.471) = 0.689. Finally, apply Raoult's Law: P_solution = (X_CHCl3 * P_CHCl3) + (X_CH2Cl2 * P_CH2Cl2) = (0.311 * 200) + (0.689 * 415) = 285.5 mm Hg.

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