Question

1) Consider two solutions, one formed by adding 10 g of glucose to 1L of water and the other formed by adding 10g of sucrose to 1L of water. Calculate the vapor pressure for each solution at 20oZC; the vapor pressure of pure water at this temperature is 17.5 torr.

Answer 1

Glucose solution :

mass of glucose = 10 g

moles of glucose = 10 / 180 = 0.055 = n1

desnity of water = 1 g / mL

volume of water = 1L = 1000 mL

mass of water = 1 x 1000 = 1000g

moles of water = 1000 / 18 = 55.55 = n2

mole fraction of glucose = n1 / n1 + n2 = 0.055 / 0.055 + 55.55 = 9.89 x 10^-4

from raoult's law

relative lowering vapour pressure of solution = mole fraction of solute

Po - Ps / Po = 9.89 x 10^-4

17.5 - Ps / 17.5 = 9.89 x 10^-4

Ps = 17.48 torr

vapour pressure of solution = 17.48 torr

sucrose solution:

n1 = 10 / 342 = 0.029

17.5 - Ps / 17.5 = 0.029 / 0.029 + 55.55

Ps = 17.49 torr

vapour pressure of solution = 17.49 torr