The question, as presented, is incomplete. The entire question is shown below:
"The density of a 13.2 mass % solution of NaF is 1.21 g/mL. Find the molarity of the solution."
We are asked to find the molarity of this solution, and molarity is defined as the amount of moles of a compound per unit volume, typically in units of mol/L.
The density of the NaF solution is 1.21 g/mL. Let's take a 1 L sample of this solution and determine the mass.
1.21 g/mL x 1000 mL = 1210 g of solution.
We are told that this solution is 13.2% NaF by mass. So the mass of NaF in this solution is:
1210 x 0.132 = 160 g NaF
We can then turn this mass of NaF into moles of NaF using the molecular weight of 42 g/mol.
160 g NaF / 42 g/mol = 3.80 moles NaF.
Now to convert this into molarity, we simply divide the amount of moles by the volume of sample, in this case 1L:
3.80 moles NaF/ 1L = 3.80 mol/L NaF or 3.80 M NaF.
The molarity is 3.80 M NaF.