How many grams of aspirin, C9H8O4, a nonvolatile, nonelectrolyte (MW = 180.1 g/mol), must be added to 258.6 grams of chloroform to reduce the vapor pressure to 171.38 mm Hg?

Question

The vapor pressure of chloroform is 173.11 mm Hg at 25°C.

How many grams of aspirin, C9H8O4, a nonvolatile, nonelectrolyte (MW = 180.1 g/mol), must be added to 258.6 grams of chloroform to reduce the vapor pressure to 171.38 mm Hg?

chloroform = CHCl3 = 119.40 g/mol.

Mass = _______ g

Answer 1

Similar to your previous question on vapor pressure. Recall that vapor pressure of a solution containing a non volatile solute is the vapor pressure of the pure solvent x the mole fraction of the pure solvent.

mols CHCl3 / mols CHCl3 + mols aspirin = mol fraction CHCl3

(mols CHCl3 / mols CHCl3 + mols aspirin) x 173.11 mm Hg = 171.38 mm Hg

mols CHCl3 = 258.6 g x 1 mol / 119.40 g = 2.166 mols CHCl3

2.166 / (2.166 + x) x 173.11 = 171.38

x = 0.02171 mols aspirin

Mass = 0.02171 mols x 180.1 g / mol = 3.910 g

(be sure to check the math)