J.R. S. answered 08/28/20
Ph.D. University Professor with 10+ years Tutoring Experience
1 L of solution x 1000 ml/L x 1.21 g/ml = 1210 g of solution.
5.01 mol/L NaOH x 40 g/mol = 200.4 g of NaOH
mass of water = 1210 g - 200.4 g = 1009.6 g
moles water = 1009.6 g x 1 mol/18 g = 56.1 moles water
Total moles = 5.01 + 56.1 = 61.11 = 61.1
mole fraction NaOH = 5.01/61.1 = 0.0820