Question

24. Calculate the vapor pressure of a 20 % by mass sucrose (MW= 342.30 g/mol) solution. The vapor pressure of pure water is 23.8 torr.

    A) 23.50 torr B) 1.40 torr C) 23.8 torr D) 0.987 torr E) 4.44 torr

Answer 1

Sol.

As 20 % by mass sucrose solution means 20 g of sucrose present in 100 g of solution

So , Mass of sucrose = 20 g

Molar Mass of sucrose = 342.30 g/mol

Moles of sucrose = 20 / 342.30 = 0.0584 mol

Mass of water = 100 - 20 = 80 g

Molar Mass of water = 18.01 g/mol

Moles of water = 80 / 18.01 = 4.4420 mol  

Then , Mole fraction of water

= Moles of water / ( Moles of water + Moles of sucrose )

= 4.4420 / ( 4.4420 + 0.0584 )

= 4.4420 / 4.5004

= 0.987  

Vapour Pressure of pure water = 23.8 torr

Therefore ,

Vapour Pressure of solution

= Mole fraction of water × Vapour pressure of pure water

= 0.987 × 23.8

=   23.50 torr ( option A )