A scientist wants to make a solution of tribasic sodium phosphate, Na3PO4, for a laboratory experiment. How many grams of Na3PO, will be needed to produce 300 mL of a solution that has a concentration of Nations of 1.50 M?
mass of NazPO4 =

molar mass Na3PO4 = 164 g / mol

Volume = 300 ml x 1 L / 1000 mls = 0.3 L

[Na+] = 1.50 M = 1.50 mol / L

mols Na+ = 1.50 mol / L x 0.3 L = 0.45 mols Na+

Since you have 3 mols Na+ for each mol of Na3PO4, the mass of Na3PO4 can be found as follows:

0.45 mol Na+ / L x 1 mol Na3PO4 / 3 mol Na+ x 164 g Na3PO4/mol Na3PO4 = 24.6 g Na3PO4 needed