Naturally occurring copper consists of isotopes of ${}^{63}\mathrm{Cu}$ and ${}^{65}\mathrm{Cu}$in the ratio $8:3.$ What will be the mean atomic weight of copper? 

Mean atomic weight is calculated by considering the ratio and atomic weight of ${}^{63}\mathrm{Cu}$and  ${}^{65}\mathrm{Cu}$

$\text{ Mean at. wt. of }\mathrm{Cu}=\frac{8\times 63+3\times 65}{8+3=11}$

$\mathrm{Mean} \mathrm{at}. \mathrm{wt}. \mathrm{of} \mathrm{Cu}=\frac{504+195}{11}=\frac{699}{11}=63.54$