# Question #e76e2

If its 1.4% by mass, then that means there are $0.014 \cdot 200.0 = 2.8 m g$ of the 214 isotope.
$2.8 m g = 0.0028 g$
$\frac{0.0028}{214} = 1.31 \cdot {10}^{- 5} m o l$
$2.8$ mg of lead-214 is therefore $1.31 \cdot {10}^{- 5}$ mol
$1.31 \cdot {10}^{- 5} \cdot 6.02 \cdot {10}^{23} = 7.9 \cdot {10}^{18}$ atoms
$1.31 \cdot {10}^{- 5}$ mol of a substance therefore contains $7.9 \cdot {10}^{18}$ atoms.