1. Calculate the mass of the formula unit (fu). Na= 22.990 g/mol Cl= 35.453 g/mol so total = 22.990 g/mol + 35.453 g/mol = 58.443 g/mol 1 mol = 6.02 x10^23 formula units, so (58.443 g/mol)*(1 mol/6.02x10^23 fu)= 9.7x10^-23 2. Convert mass to grams: (0.17 mg)*(1 g/1000 mg) = 0.00017 g 3. Calculate number of formula units: 0.00017 g x (1 fu/9.7x10^-23 g) = 1.8x10^18 Or more simply: (.00017 g) * (1 mol/58.443 g) * (6.02x10^23 fu/1mol) = 1.8x10^18