Question

Given that the vapor pressure of water is 17.54 Torr at 20 °C, calculate the vapor-pressure lowering of aqueous solutions that are 2.30 m in (a) sucrose, C12H22O11, and (b) aluminum chloride. Assume 100% dissociation for electrolytes.

Answer 1

Given that the vapor pressure of water is 17.54 torr

And molality = 2.30m

Molaity = number of moles / mass in kg

2.30 = number of moles / 1.00 kg

Number of moles of solutes = 2.30

Mole of water = amount in g / molar mass

= 1000 g/18.02

= 55.5 mole

For sucrose solution = 2.30+55.5

= 57.8 mole

Mole fraction of water =55.5/ 57.8

= 0.96

vapor pressure of solution = mole fraction * vapore pressure

= 0.960 *17.54 torr

= 16.84 torr

Vapor pressure lowering sucrose solution = 17.54 - 16.84

= 0.70 torr

For AlCl3 = Al+ 3Cl-

Total moles = 4 *2.30

= 9.2

For sucrose solution = 9.20+55.5

= 64.7 mole

Mole fraction of water =55.5/ 64.7

= 0.858

vapor pressure of solution = mole fraction * vapore pressure

= 0.858 *17.54 torr

= 15.05 torr

Vapor pressure lowering sucrose solution = 17.54 – 15.05

= 2.49 torr