Question

Calculate molarity of 10% NaOH (w/v). Also find mole fraction and molality when density is 1.5gm/ml.

Answer 1

10% NaOH ie 10g NaOH in 100ml solution

Density of solution = 1.5g/ml

Mass of solution = V x Density = 100 x 1.5 = 150g

Moles of NaOH = Given mass/Molar mass = 10/40 = 0.25 mol

Mass of solvent(water) = Mass of solution - Mass of solute = 150 - 10 = 140g

Moles of water = 140/18 = 7.8 mol

Mole fraction of NaOH = no. of moles of NaOH/no. of moles of solution = 0.25/8.05 = 0.03

Mole fraction of water = 1 - 0.03 = 0.97

Molality = no. of moles of NaOH/Mass of solvent(water) in Kg = 0.25/0.14 = 1.78 mol/Kg