How many grams of a 23.4% by mass naf solution is needed if you want to have 1.33 moles of naf?

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