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Question

Calculate the molality of a 2 M aqueous salt solution in a solvent X, if the mole fraction of the salt in the solution is 0.5.
(Molar mass of solvent = 40 g/mol)

A
8 m
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B
14.5 m
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C
7.4 m
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D
25 m
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Solution

The correct option is D 25 m
2 molar solution means 2 mol of soluble salt is present in 1 L of the solution.
Mole fraction of solute =xsolute=moles of solutemoles of solute + moles of solvent
0.5=22+nX
On solving,
Moles of solvent X =nX=2 mol
Weight of solvent =2×40=80 g
Molality (m)=moles of solutemass of solvent in g×1000
Molality=280×1000=25 m

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