Question

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

• A. 0.39 g
• B. 0.37 g
• C. 22.05 g
• D. 2.2 g

Answer 1

Answer: (C)

$\left[F{e}^{2+}⟶F{e}^{3+}+e^{-}\right]\times 6$
From Mohr's salt
$C{r}_2{O}_7^{2-}+14H^{+}+6e^{-}\to C{r}^{3+}+7H_{2}O$
$6F{e}^{2+}+C{r}_2{O}_7^{2-}+14H^{+}\to 6F{e}^{3+}+2C{r}^{3+}+7H_{2}O$
Millimoles of $F{e}^{2+}=750\times 0.6=450$
Moles of $F{e}^{2+}=\frac{450}{1000}$
$=0.450=0.450mol$
$6molF{e}^{2+}\equiv 1molC{r}_2{O}_7^{2-}$
$\therefore 0.450molF{e}^{2+}=\frac{0.450}{6}=0.075molC{r}_2{O}_7^{2-}$
$=0.075\times 294g$
$\Rightarrow =22.05g$