Answer
$3.982\times10^{22}\ atoms\ Pt $
Work Step by Step
1. Use the molar mass of platinum and Avogadro's number as conversion factors in order to determine the number of atoms of platinum.
2. $12.899 g\ Pt \times\frac{1\ mol\ Pt}{195.08\ g\ Pt}\times\frac{6.022\times10^{23}}{1\ mol\ Pt}= 3.982\times10^{22}\ atoms\ Pt $
