An ideal solution is formed from a mixture of the nonvolatile solute urea, co(nh2)2, and methanol, ch3oh. the vapor pressure of pure methanol at 20°c is 89 mmhg. if 6.0 g of urea is mixed with 39.8 g of methanol, calculate the vapor pressure of the methanol solution.

When a non volatile solute is added to a solvent, there is a lowering of vapour pressure. The relation between relative lowering vapour pressure and mole fraction is

relative lowering of vapour pressure = mole fraction of solute

moles of urea taken = mass / molar mass = 6 / 60 = 0.1 moles

Moles of methanol = mass / molar mass = 39.8 / 32.04 = 1.24

So total moles  = 0.1 + 1.24 = 1.34

Mole fraction of urea = 0.1 / 1.34 = 0.075

Relative lowering of vapour pressure =

Ps = vapour pressure of solution

P0s = vapour pressure of pure solvent

0.075 = Ps / 89 - Ps

6.675 - 0.075Ps = Ps

6.675 = 1.075Ps

Ps = 6.21 mmHg

it is the most commonly accepted one