$6.4\times {10}^{31} \mathrm{atoms} \mathrm{of} \mathrm{iron} \mathrm{weighs}$

1 mole of iron has $6.023\times {10}^{23}$ iron atoms and it weighs 55.845 g.

We need to find the weight of $6.4\times {10}^{31}$ iron atoms.

$\frac{55.845 \mathrm{g} \times 6.4\times {10}^{31} \mathrm{atoms}}{6.023\times {10}^{23} \mathrm{atoms}}=59.73\times {10}^{8}g$