The density of $3 M$ solution of $N a C l$ is $1.25 g m L^{- 1}$ . Calculate molality of the solution.

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Solution

The volume of the solution be $1000$ ml.

$∴$ Moles of $N a C l = 3$ moles

$∴$ Mass of $N a C l = 3 \times (23 + 35.5) g$

= $175.5$ g= $0.1755$ kg

$∴$ Density = $\frac{M a s s o f s o l u t i o n}{V o l u m e o f s o l u t i o n}$

$∴$ Mass of solution = $1.26 \times 1000$

= $1260$ g = $1.26$ kg

$∴$ Mass of solvent = Mass of solution- Mass of $N a C l$

= $1.26 - 0.1755$

= $1.0845$ kg

$∴$ Molality = $\frac{M o l e s o f N a C l}{M a s s o f s o l v e n t (k g)}$

$= \frac{3}{1.0845}$

$∴$ Molality = $2.766$ m

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