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

Molality of $\text{Na}^{\text{+}}$ ions (m)

$\text{=} \, \frac{\text{Number} \, \text{of} \, \text{moles} \, \text{of} \, \text{Na}^{\text{+}} \, \text{ions}}{\text{Mass} \, \text{of} \, \text{water} \, \text{in} \, \text{kg}}$

$=\frac{(92 \mathrm{~g}) /\left(23 \mathrm{~g} \mathrm{~mol}^{-1}\right)}{(1 \mathrm{~kg})}$

$\text{=} \, \text{4} \, \text{mol} \, \text{kg}^{- \text{1}} \, \text{or} \, \text{4} \, \text{m}$