Question

What is the molality of ammonia in a solution containing 0.85 g of NH₃ in 100 cm³ of a liquid of density 0.85 g cm⁻³?

Answer 1

Given: Mass of NH₃ = 0.85 g

Volume of solution = 100 cm³

Density of solution = 0.85 g/cm³

⇒ Mass of solution = 100 × 0.85 = 85 g

Molar mass of NH₃ = 17 g/mol

Moles of NH₃ = `0.85/17`

Moles of NH₃ = 0.05 mol

Mass of solvent = Mass of solution − Mass of solute

= 85 g − 0.85 g

= 84.15 g

Mass of solvent = 0.08415 kg

Molality (m) = `"Moles of solute"/"Mass of solvent n kg"`

= `0.05/0.08415`

= 0.594 mol/kg

∴ Molality of ammonia in a solution is 0.594 mol/kg.