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A compound H2X (molar mass 80 g/mol) is dissolved in a solvent of density 0.4 g/mL. Assuming the volume does not change on dissolving, find the molality of a 3.2 molar solution.

1. 8 m
2. 3.2 m
3. 4 m
4. 6.4 m

Correct answer: 8 m

Solution

Since volume is unchanged, 1 L of solvent (0.4 kg) holds 3.2 mol solute, but molality = 3.2/0.4 = 8 m.

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