What is the mass of 5.05*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:

5.05*10^21 atoms Pt (1 mol / 6.022 x 10^23 atoms) (195.084 g / 1 mol) = 1.64 g Pt