Question

the vapor pressure of water at 25°c is 23.8 torr. determine the mass of glucose(molar mass =180g/mol) needed to add to 500g water to change the vapor pressure to 23.1 torr.

Answer 1

Vapor pressure of solution = vapor pressure of pure water x mole fraction of water

mole fraction of water = 23.1/23.8 = 0.97

mole fraction of water = moles of water/total moles

moles of water = 500 g/18.015 g/mol = 27.755 mols

0.97 = 27.55/(27.55 + x)

x being moles of glucose

0.97x + 26.72 = 27.55

moles of glucose x = 0.85 mols                                 

mass of glucose to be added = 0.85 mol x 180 g/mol = 153 g