In a 69.0-g aqueous solution of methanol, CH4O, the mole fraction of methanol is 0.250.

Question

What is the mass of CH4O?

What is the mass of water?

Answer 1

Mole fraction CH4O = 0.250

Mole fraction H2O = 1 - 0.250 = 0.750

If we assume we have a total of 1 mole of material (H2O + CH4O), then...

0.750 moles H2O x 18 g /mol = 13.5 g H2O

0.250 moles CH4O x 32 g / mol = 8.00 g CH4O

Total grams = 21.5

But since we know that we have 69.0 g, that makes up the 1 mol, we can make the adjustment

69.0 g / 21.5 g = 3.2, so we will multiply the mass of each by this factor to get ...

13.5 g H2O x 3.2 = 43.2 g H2O

8.00 g CH4O x 3.2 = 25.6 g CH4O