To find mass of a bunch of atoms, we first have to convert to moles (using Avogadro's number), then the atomic mass (from the periodic table):
5.33 x10^21 atoms Pt x (1 mole/6.022x10^23 atoms) x (195.08 grams Pt/1 mole)
= 1.73 grams of Pt
Notice the answer has three digits - three significant figures since the 5.33 has three sig figs - the fewest of all the values in the problem.
