$\underset{2g}{H _{2}+1} / \underset{16g}{2 O _{2}} \rightarrow \underset{18g}{H _{2} O}$
$10\, g\, H _{2}$ required $O _{2}=80$ which is not present $64\, g\, O _{2}$ required $8\, g$ of $H _{2}$ and $H _{2}$ left $=2 g$. Thus, $O _{2}$ is the limiting reactant and $H _{2}$ is excess reactant.
Hence, $H _{2} O$ formed from 64 of $O _{2}$
$=\frac{18}{16} \times 64$
$=72\, g =\frac{72}{18}\, mole$
$=4$ mole