Question

If the density of some lake water is 1.25 g mL⁻¹ and contains 92 g of Na⁺ ions per kg of water, calculate the molality of Na⁺ ions in the lake.

Answer 1

Given: Mass of Na⁺ ions = 92 g

Mass of water (solvent) = 1 kg

Molar mass of Na⁺ = 23 g/mol

Number of moles present in 92 g of Na⁺ ions = `92/23`    ...(as atomic mass of Na = 23)

= 4 mole

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

= `4/ 1`

= 4 mol/kg

Hence, the molality of lake water is 4 m.