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Question

Calculate the molality of a 1.5 M aqueous salt solution in a solvent X, if the mole fraction of the salt in the solution is 0.25.
(Molar mass of solvent = 40 g/mol)

A
18.52 m
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B
12.85 m
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C
8.33 m
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D
15.24 m
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Solution

The correct option is C 8.33 m
1.5 molar solution means 1.5 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
0.25=1.51.5+nX
On solving,
Moles of solvent X =nX=4.5 mol
Weight of solvent =4.5×40=180 g
Molality(m)=moles of solutemass of solvent in g×1000
Molality=1.5180×1000=8.33 m

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