# How many moles do 8.1xx10^18 calcium atoms represent?

Approx. $\frac{1}{100000} \text{th}$ of a mole.
In $1$ $m o l$ of particles there are $6.022$ $\times$ ${10}^{23}$ individual particles. So we just divide the quoted number by this, Avogadro's number, to get an answer in mol.
$\left(8.1 \times {10}^{18} \text{ calcium atoms")/(6.022xx10^23 " calcium atoms} \cdot m o {l}^{-} 1\right)$ $=$ ?? $m o l$