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# If the density of some lake water is $1.25g mL^{–1}$ and contains 92 g of $Na^+$ ions per kg of water, calculate the molality of $Na^+$ ions in the lake.

$\begin{array}{1 1}4m\\5m\\6m\\7m\end{array}$

Number of molality = $\large\frac{\text{Mole of }Na^+\text{ ions}}{\text{Mass of water(kg)}}$
92g of $Na^+$ ions =$\large\frac{929}{23gmol^{-1}}$
$\Rightarrow 4$ mole
Therefore molality = 4m