Question

Calculate molality of 1 litre solution of 90% H₂SO₄ (weight/volume). The density of solution is 1.80 g mL⁻¹.

Answer 1

Given: Strength of H₂SO₄ solution = 90% w/v → 90 g of H₂SO₄ in 100 mL of solution

Volume = 1 litre = 1000 mL

Density = 1.80 g/mL

Molar mass of H₂SO₄ = 98 g/mol

Mass of solute in 1000 mL from 90 g per 100 mL = 90 × 10 = 900 g

Mass of total solution = Volume × Density 

= 1000 × 1.80

= 1800 g

Mass of solvent (water) = 1800 − 900

= 900 g 

= 0.900 kg

Moles of H₂SO₄ = `900/98`

= 9.18 mol

Molality (m) = `9.18/0.900`

= 10.2 mol/kg

∴ The molality of 1 litre solution of 90% H₂SO₄ is 10.2 mol/kg.