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Formula used: number of moles$ = \dfrac{m}{M}$

molality$ = \dfrac{n}{{{M_s}}}$

Where $m$ is given mass of substance, $M$ is molecular mass of substance, $n$ is number of moles,${M_s}$ is mass of solvent in kilogram

As we know the density of a substance is mass per unit volume of that substance and molality is the number of moles present per kilogram of solvent.

In this question we have given:

Density of lake water$ = 1.25gm{L^{ - 1}}$

Mass of sodium ions $\left( m \right) = 92g$

Mass of solvent $\left( {{M_s}} \right) = 1Kg$

We know molecular mass of sodium $\left( M \right)$ is $23g$

Therefore number of moles of sodium ions $\left( n \right)$ is $\dfrac{m}{M}$

$\

n = \dfrac{{92}}{{23}} \\

n = 4 \\

\ $

Now, we know $n = 4$ and ${M_s} = 1$ and we know formula to calculate molality (written above)

Molality of sodium ions$ = \dfrac{n}{{{M_s}}}$

molality$ = \dfrac{4}{1} = 4m$

So, the molality of sodium ions is $4m$.

The terms molality and molarity are different. Molality is the measure of a number of moles present per kilogram of solvent and molarity is the measure of a number of moles present per liter of solution. In molality mass of the solvent is used and in molarity volume of solution is used.

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