What is the mass of 5.31*1021 platinum atoms?

Avogadro's number represents the number of units in one mole of any substance. This has the value of 6.022 x 10^23 units / mole. This number can be used to convert the number of atoms or molecules into number of moles. We calculate as follows:

5.31*10^21 platinum atoms (1 mol / 6.022 x 10^23 atoms) (78 grams / 1 mol) = 0.6878 grams platinum