The mole fraction of water in a solution is 0.9. If moles of water are 9, how many moles of solute are present?

Correct answer: 1.0 mol

Mole fraction of water is the ratio of water moles to total moles. Let the solute amount be x mol. Then 9/(9 + x) = 0.9. Solving gives 9 = 8.1 + 0.9x, so 0.9 = 0.9x and x = 1.0 mol. Equivalently, 9 mol is 90% of the total, so total moles are 10 and solute moles are 1. Option B is correct.