How many oxygen atoms are in 3.00 grams of sodium dichromate, Na2Cr2O7?

The solution is as follows: Convert 3.00 g Na2Cr2O7 into mol. To do this, we must know the molar mass. Molar mass of Na2Cr2O7 is 261.97 g/mol Mol Na2Cr2O7 = (3.00 g Na2Cr2O7) (mol / 261.97 g) = 0.01145 mol In 1 mol of Na2Cr2O7 there are 7 mol of O (oxygen). Therefore: Mol O = 0.01145 mol Na2Cr2O7 (7 mol O / 1 mol Na2Cr2O7) = 0.08016 mol O In 1 mol of any substance, there are 6.02 x 10^23 atoms Atoms of O = 0.08016 mol O (6.02 x 10^23 atoms / mol O) = 4.8257 x 10^22 atoms