percentage by mass is the mass of NaF present in 100 g of the solution. the percentage by mass of NaF is 23.4 % this means that in 100 g of solution, mass of NaF present is 23.4 g the number of moles of NaF present - 1.33 mol mass of NaF - 1.33 mol x 42 g/mol = 55.9 g when there's 23.4 g of NaF - mass of solution is 100 g therefore when there's 55.9 g of NaF - mass of solution is 100 / 23.4 x 55.9 = 239 g mass of solution required is 239 g