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A 5% solution of cane sugar (molar mass 342) is isotonic with a 1% solution of an unknown solute. What is the molar mass of the unknown solute in g mol−1?

1. 171.2
2. 68.4
3. 34.2
4. 136.2

Correct answer: 68.4

Solution

Isotonic solutions have equal molarity, so (5/342) = (1/M), giving M = 342/5 = 68.4 g mol^-1.

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