A KCl solution is prepared by dissolving 20.0 g KCl in 250.0 g of water at 25°C.What is the vapor pressure of the solution if the vapor pressure of water at 25°C is 23.76 mm Hg?
A)22.0 mm Hg
B)22.9 mm Hg
C)23.3 mm Hg
D)24.6 mm Hg

Question

Quizplus · Accepted Answer

The vapor pressure of a solution is always lower than the vapor pressure of the pure solvent. This difference in vapor pressure is known as the vapor pressure lowering. The vapor pressure lowering of a solution is proportional to the mole fraction of solute particles in the solution. We can use the following formula to calculate the vapor pressure of the solution:

Psolution = Xsolvent * Psolvent

where Psolution is the vapor pressure of the solution, Xsolvent is the mole fraction of the solvent, and Psolvent is the vapor pressure of the pure solvent.

First, we need to calculate the mole fraction of water in the solution:
moles KCl = 20.0 g / 74.55 g/mol = 0.268 mol
moles water = 250.0 g / 18.015 g/mol = 13.87 mol
Xwater = moles water / (moles water + moles KCl) = 13.87 mol / 14.14 mol = 0.98

Next, we can use the mole fraction of water and the vapor pressure of water to calculate the vapor pressure of the solution:
Psolution = Xwater * Psolvent
Psolution = 0.98 * 23.76 mmHg = 23.3 mmHg

Therefore, the vapor pressure of the solution is 23.3 mmHg, and the best choice is C.

A KCl Solution Is Prepared by Dissolving 20