A salt crystal has a mass 0.14mg, How many NaCl formula units does it contain?

Express your answer using two significant figures.

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