You have a solution which is 32.2% by mass isopropyl alcohol (MM = 60.11 g/mol) in water. The density of this solution is 0.905 g/mL. What is the molality of the isopropyl alcohol in the solution?

Answer 1

Molality (m) = moles of solute per kg of solvent.

A 32.3% by mass IPA means 32.2 g of IPA per 100 g of solution. Thus in 100 g of solution, the mass of water (solvent) = 100g - 32.2 g = 67.8 g of water = 0.0678 kg

moles of IPA = 32.2 g x 1 mol/60.11 g = 0.536 mols

Molality = 0.536 mols/0.0678 kg = 7.90 m

(Density of solution isn’t required unless you wish to determine the molarity).