Question

Determine the vapor pressure of a solution at 55 °C that contains 34.2 g NaCl in 375 g of water. The vapor pressure of pure water at 55 °C is 118.1 torr. The van't Hoff factor for NaCl is 1.9

Answer 1

Vapor pressure of solution = 111.98 Torr

Step-by-step explanation:

Colligative property to apply: Lowering vapor pressure

P° - P' = P° . Xm . i

P°, vapor pressure of pure solvent

P', vapor pressure of solution

Xm, mole fraction of solute

i, Van't Hoff factor.

Let's determine the Xm.

Moles of solute = mass / molar mass → 34.2 g / 58.45 g/mol = 0.585 moles

Moles of solvent = 375 g / 18 g/mol = 20.83 moles

Mole fraction = 0.585 mol / 0.585 mol + 20.83 mol → 0.0273

Let's replace the data in the formula:

118.1 Torr - P' = 118.1 Torr . 0.0273 . 1.9

118.1 Torr - 6.12 Torr = P'

Vapor pressure of solution = 111.98 Torr