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In a solution the mole fraction of the solute is 0.1. At 298 K the solution's molarity equals its molality, and its density is 2.0 g/cm³. Find the ratio of the molecular weights of solute to solvent (MW_solute / MW_solvent).

1. 9
2. 10
3. 18
4. 0.1

Correct answer: 9

Solution

Equating molarity and molality with density 2.0 gives MW_solute = 9*MW_solvent, so the ratio is 9.

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