Determine the vapor pressure of a solution at 55°C that contains 34.2 g NaCl in 375 mL of water.The vapor pressure of pure water at 55°C is 118.1 torr.
A)115 torr
B)87.1 torr
C)108 torr
D)112 torr
E)92.8 torr

Question

Quizplus · Accepted Answer

Raoult's law states that the vapor pressure of a solution is equal to the mole fraction of the solvent multiplied by the vapor pressure of the pure solvent. The mole fraction of the solvent is equal to the number of moles of the solvent divided by the total number of moles of the solution. The number of moles of the solvent can be calculated by dividing the mass of the solvent by its molar mass. The number of moles of the solute can be calculated by dividing the mass of the solute by its molar mass. The total number of moles of the solution is equal to the sum of the number of moles of the solvent and the number of moles of the solute.In this problem, the mass of the solvent is 375 mL, the density of water is 1 g/mL, so the mass of the solvent is 375 g. The molar mass of water is 18 g/mol, so the number of moles of the solvent is 375 g / 18 g/mol = 20.83 mol. The mass of the solute is 34.2 g, and the molar mass of NaCl is 58.44 g/mol, so the number of moles of the solute is 34.2 g / 58.44 g/mol = 0.585 mol. The total number of moles of the solution is 20.83 mol + 0.585 mol = 21.42 mol. The mole fraction of the solvent is 20.83 mol / 21.42 mol = 0.972. The vapor pressure of the pure solvent is 118.1 torr, so the vapor pressure of the solution is 0.972 * 118.1 torr = 115 torr.Therefore, the correct answer is A.

Determine the Vapor Pressure of a Solution at 55°C That