Question

The mass of potassium dichromate crystals required to oxidise $750 \,cm ^{3}$ of $0.6\, M$ 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)

$\left[ Fe ^{2+} \longrightarrow Fe ^{3+}+ e ^{-}\right] \times 6$
From Mohr's salt
$ Cr _{2} O _{7}^{2-}+14 H ^{+}+6 e ^{-} \to Cr ^{3+}+7 H _{2} O$
$6 Fe ^{2+}+ Cr _{2} O _{7}^{2-}+14 H ^{+} \to 6 Fe ^{3+}+2 Cr ^{3+}+7 H _{2} O$
Millimoles of $Fe ^{2+}=750 \times 0.6=450$
Moles of $Fe ^{2+} =\frac{450}{1000} $
$=0.450=0.450\, mol $
$ 6 \,mol \,Fe ^{2+} \equiv 1 \,mol \,Cr _{2} O _{7}^{2-} $
$\therefore 0.450\, mol \,Fe ^{2+} =\frac{0.450}{6}=0.075\, mol\, Cr _{2} O _{7}^{2-} $
$=0.075 \times 294 \,g $
$\Rightarrow =22.05 \,g $