15 mL of aqueous solution of $\mathrm{Fe}^{2+}$ in acidic medium completely reacted with 20 mL of 0.03 M aqueous $\mathrm{Cr}_{2}\mathrm{O}_{7}^{2-}$. The molarity of the $\mathrm{Fe}^{2+}$ solution is ______ $\times 10^{-2}$ M (Round off to the nearest integer).

JEE Main · 2021 · 17 Mar 2021 · Shift 1 · Redox Reactions and Electrochemistry

15 mL of aqueous solution of

Fe^{2 +}

in acidic medium completely reacted with 20 mL of 0.03 M aqueous

Cr_{2} O_{7}^{2 -}

. The molarity of the

Fe^{2 +}

solution is ______

\times 1 0^{- 2}

M
(Round off to the nearest integer).

Answer: 24

Dichromate oxidises iron(II) in acid:

C r_{2} O_{7}^{2 -} + 6F e^{2 +} + 14 H^{+} \to 2C r^{3 +} + 6F e^{3 +} + 7 H_{2} O

The dichromate taken is

0.03 \times 0.020 = 6 \times 1 0^{- 4}

mol, and six moles of iron(II) are consumed per mole of dichromate, so the iron(II) present is

6 \times 6 \times 1 0^{- 4} = 3.6 \times 1 0^{- 3}

mol. This was contained in 15 mL, so

M = \frac{3.6 \times 1 0 ^{- 3}}{0.015} = 0.24 M

and the molarity of the iron(II) solution is

24 \times 1 0^{- 2}

JEE Main · 2021 · 17 Mar 2021 · Shift 1 · Integer answer