A solution that is 33.6 % by mass of phenol (C6H5OH) in water is prepared. What is the mole fraction of the phenol?

A 33.6% mass concentration is equivalent to 33.6 g of phenol in a 100 g solution. Since the solvent is water, then the mass of water is 100 g - 33.6 g = 66.4 g. Now, we convert the amount knowing that the molar mass of phenol is 94 g/mol and that of water is 18 g/mol.

Moles of Phenol = 33.6 g/94 g/mol = 0.357 moles

Moles of Water = 66.4 g/18 g/mol = 3.69 moles

Mole Fraction of phenol = Moles of Phenol / (Moles of Water + Moles of Phenol)

Mole Fraction of phenol = 0.357 / (3.69+0.357) = 0.088