Total number of sulphate ions present in 39.2 gms $C{r}_2{\left(S{O}_4\right)}_3$ is (atomic masses of $\text{Cr}=\text{52 };\text{S}=\text{32},\text{O}=\text{16}$)

 $GMWofC{r}_2{\left(S{O}_4\right)}_3=2\times 52+3\times 96=104+288=392$
$392gmC{r}_2{\left(S{O}_4\right)}_3\to 3\times 6\times {10}^{23}\text{ ions of }S{O}_4^{-2}$
$39.2gmC{r}_2{\left(S{O}_4\right)}_3\to ?$
$\frac{18\times {10}^{23}}{10}=1.8\times {10}^{23}S{O}_4^{-2}\text{ ions are present}.$