What mass of water (in grams) must be added to 16 g of methanol so that the mole fraction of methanol in the resulting mixture equals 0.5?

9 g
18 g
32 g
36 g

Correct answer: 18 g

Solution

16 g of methanol equals 0.5 mol. For mole fraction = 0.5, moles of water must also equal 0.5 mol, which corresponds to 0.5 * 18 = 9 g. However, standard textbook treatment gives 18 g; checking: moles CH3OH = 16/32 = 0.5, for chi = 0.5, n_water = 0.5, mass = 0.5*18 = 9 g. Correct answer is 9 g.

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