What is the mole fraction of NaOH in an aqueous solution that contains 15% NaOH by mass?

0.074

Explanation:

15% means that in 100 g of solution 15 g sodium hydroxide is present.

Mass of water = 100 - 15 = 85 g

Number of moles of sodium hydroxide:

Number of moles = 15 g/40 g/mol

Number of moles = 0.375 mol

Number of moles of water:

Number of moles = 85 g/18 g/mol

Number of moles = 4.7 mol

Moles fraction of NaOH:

moles of NaOH / moles of solvent + moles of solute

0.375 mol / 0.375 mol+4.7mol

0.375 mol / 5.075 mol

0.074