Density of 3 M NaCl solution is 1.25 g/mL. The molality of the solution is: [2025]

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Solution

Q.
Density of 3 M NaCl solution is 1.25 g/mL. The molality of the solution is: [2025]

1 2 m

2 2.79 m

3 1.79 m

4 3 m

Ans.
(2)

Molar mass of NaCl = 23 + 35.5 = 58.5 g ${mol}^{- 1}$

3 M NaCl solution has:

3 mol NaCl in 1000 mL solution

$i.e 3 \times 58.5 g NaCl in (1000 mL \times 1.25 {g mL}^{- 1}) solution$

$i . e$ $175.5 g NaCl in 1250 g solution$

$i . e$ $175.5 g NaCl in (1250 - 175.5) g solvent$

$i . e$ $175.5 g NaCl in 1074.5 g solvent$

$i . e$ $3 mol NaCl in 1074.5 g solvent$

Molality (m) : $= \frac{n_{solute}}{W_{solvent} (in g)} \times 1000$

$= \frac{3}{1074.5} \times 1000 = 2.79 m$

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