What is the vapor pressure of isopropanol (MW = 60.1) in a closed container of 70% isopropanol in water (MW = 18.0) at 25oC? Such a mixture contains 70 g of isopropanol and 30 g of water. The vapor pressure of pure isopropanol at 25oC = 50 torr

The vapor pressure of the 70% isopropanol solution is 21 Torr.

Explanation:

Relative lowering of vapor pressure for solution with non volatile solute is given by :

Where:

= Vapor pressure of pure solvent

p = Vapor pressure of solution

=  Mole fraction of solute

We have, 70% isopropanol solution which menas 70 grams of isopropanol (solvent) and 30 grams of water (solute).

Mass of water = 30 g

Moles of water =

Mass of isopropanol = 70 g

Moles of isopropanol =

Mole fraction of solute that is water :

Vapor pressure of the pure isopropanol =

Vapor pressure of the solution = p

Solving for p:

p = 20.57 Torr ≈ 21 Torr

The vapor pressure of the 70% isopropanol solution is 21 Torr.