# How many lanthanum atoms are in 69.5g lanthanum?

There are $3.01 \times {10}^{23}$ atoms of La in $\text{69.5 g La}$.
The given mass of lanthanum (La) must be converted into moles. One mole of atoms is $6.022 \times {10}^{23}$ atoms. The molar mass of La is $\text{138.90547 g/mol}$ (from periodic table).
To determine mol La, divide the given mass by its molar mass. Since molar mass is a fraction, $\text{g"/"mol}$, multiply the given mass by the inverse of the molar mass. Then multiply by $6.022 \times {10}^{23}$ atoms/mol.
69.5color(red)cancel(color(black)("g La"))xx(1color(red)cancel(color(black)("mol La")))/(138.90547color(red)cancel(color(black)("g La")))xx(6.022xx10^23"atoms La")/(1color(red)cancel(color(black)("mol La")))=3.01xx10^23" atoms La"