Question

What is the vapor pressure in mmHg of a solution made by dissolving 12.5 g of NaCl in 500.0 g of water at 70°C, assuming a van't Hoff factor of 1.9? The vapor pressure of pure water at 70°C is 233.7 mmHg. watch for sig figs

Answer 1

moles of NaCl = 12.5 g / 58.44 (g/mol) = 0.2139 mol

moles of H2O = 500 g / 18 (g/mol) = 27.778 mol

now

given vanthoff factor of NaCl is 1.9

so

effective moles of NaCl = 1.9 x 0.2139 = 0.40641 mol

now

mole fraction of water = moles of water / ( moles of water + effective moles of NaCL)

mole fraction of water = 27.778 / ( 27.778 + 0.40641)

mole fraction of water = 0.9856

now

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

vapor pressure of solution = 0.9856 x 233.7 mmHg

vapor pressure of solution = 230.33 mm Hg