Question

Calculate the molal (not molar) concentration:

c) A 1% (w/w) solution of cocaine hydrochloride (molar mass 339.81 g/mol) in water.

Answer 1

1 % w/w means 1 g of solute in 100 g of solution.

Hence,

mass water = 100 g – 1 g = 99 g = 0.099 kg

moles of cocaine hydrochloride = mass / molar mass = 1 g / 339.81 g/mol = 2.94 x 10^-3 mol

molality = moles / mass of solvent in kg

             = 2.94 x 10^-3 mol / 0.099 kg

             = 0.03 m

Therefore,

Molal concentration = 0.03 m