To determine the number of formula units in a sample of a compound you need to divide the number of grams by the formula mass. The formula mass of NaCl is 23 g/mol + 35.5 g/mol = 58.5 g/mol, and the number of grams of the sample is 0.14 mg 1 g/ 1000 mg = 0.00014 g. Then the answer is 0.00014 g / 58.5 g = 2.30 10^ -6, which rounded to two significant figures is 2.4 10^ -6. So the answer is 2.4 10^-6 or 0.0000024
