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 molarity 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 mol

Volume of solution = `"Mass of solution"/"Density of solution"`

= `1000/1.25`

= 800 mL

= 0.8 L

Molarity = `("Number of moles Na"^+ "ions")/"Volume of solution in liter"`

= `4/0.8`

= 5 mol L⁻¹

= 5 m