210. moles To determine the number of moles of a substance you have, simply divide the mass of the substance you have by its molar mass. So 3.79 kg / 18.02 g/mol = 3790 g / 18.02 g/mol = 210.3218646 mol Since we only have 3 significant figures, the answer should be rounded to 3 figures, giving 210. as the result. Notice that the decimal point is required. If you state the number of moles is 210, then that answer will only have 2 significant figures. But the value of 210. has 3 significant figures.