Question 5
40 g of NaOH is dissolved in 180 g of water. What is the mole fraction of the sodium hydroxide? (H = 1, O = 16, Na = 23)
The right answer is the one marked above.
1 ÷ 11. Water: 180 g ÷ 18 g mol⁻¹ = 10 mol. NaOH: 40 g ÷ 40 g mol⁻¹ = 1 mol. So the mole fraction is 1 ÷ (10 + 1).
10 ÷ 11 is the mole fraction of the water, and the two add to 1 — which is the quickest check on any mole-fraction answer. Dividing the masses directly skips the step that turns grams into particles.