The mass of potassium dichromate crystals required to oxidize  $750c{m}^3$ of 0.6 M Mohr’s  salt solution is (Given, molar mass: potassium dichromate=294, Mohr’s salt=392)

Mohr’s salt is  $FeS{O}_4.{\left(N{H}_4\right)}_{2}S{O}_4.6H_{2}O$
Only oxidisable part is  $F{e}^{2+}$
$\left[F{e}^{2+}\to F{e}^{3+}+e^{-}\right]\times 6$ 
 $C{r}_2{O}_7^{2-}+14H^{+}+6e^{-}\to 2C{r}^{3+}+7H_{2}O$
 $6F{e}^{2+}+C{r}_2{O}_7^{2-}+14H^{+}\to 6F{e}^{3+}+2C{r}^{3+}+7H_{2}O$
Millimoles of  $F{e}^{2+}=750\times 0.6=450mmol$
Moles of  $F{e}^{2+}=\frac{450}{1000}=0.450mol$
6 moles of  $F{e}^{2+}\equiv 1moleofC{r}_2{O}_7^{2-}$
0.450 mole of   $F{e}^{2+}\equiv \frac{0.450}{6}=0.0075$  mole of  $C{r}_2{O}_7^{2-}$.
 $=0.075\times 294g=22.05g$