Question

Raoult’s Law Calculations

The vapor pressure of diethyl ether (ether) is 463.57 mm Hg at 25°C.

How many grams of aspirin, C₉H₈O₄, a nonvolatile, nonelectrolyte (MW = 180.1 g/mol), must be added to 224.2 grams of diethyl ether to reduce the vapor pressure to 458.89 mm Hg ?

diethyl ether = CH₃CH₂OCH₂CH₃ = 74.12 g/mol.

g aspirin

Answer 1

By Raoult's law

P = X(Po)

where X = mole fraction

P0 = vapour pressure of the pure solvent = 463.57 mmHg

P is the valour pressure of the solution = 458.89 mmHg

put all these in the above equation calculate the mole froaction

458.89 mmHg = X (463.57 mmHg)

X = 458.89 / 463.57

X = 0.99

X = moles of the solvent / moles of the solvent + moles of the solute

moles of the solvent = weight / molar mass of the solvent = 224.2 / 74.12 = 3.025 moles

0.99 = 3.025 / 3.025 + moles of the solute

3.025 + moles of the solute = 3.025 / 0.99

3.025 + moles of the solute = 3.055

moles of the solute = 3.055 - 3.025   = 0.0303

weight of the solute = moles of the solute x molar mass of the solute

= 0.0303 moles x 180.1 grams/mole

= 5.47 grams