Total number of sulphate ions present in 3.92g of chromic sulphate is (Cr = 52, S=32, O=16)

mol. wt of  $C{r}_2\left(S{O}_4\right)$ is 392

$C{r}_2{\left(S{O}_4\right)}_3\to 2C{r}^{3+}+3S{O}_4^{2-}$

392 gas of  $C{r}_2{\left(S{O}_4\right)}_3$ contains 3N sulphate ions

3.92gms of  $C{r}_2{\left(S{O}_4\right)}_3$  contain $\frac{3\times 6\times {10}^{23}\times 3.92}{392}=1.8\times {10}^{22}S{O}_4^{2-} ions$