When 10 ml of an aqueous solution of \(\mathrm{Fe}^{2+}\) ions was titrated in the presence of dil \(\mathrm{H}_{2} \mathrm{SO}_{4}\) using diphenylamine indicator, 15 mL of 0.02 M solution of \(\mathrm{K}_{2} \mathrm{Cr}_{2} \mathrm{O}_{7}\) was required to get the end point. The molarity of the solution containing \(\mathrm{Fe}^{2+}\) ions is \(x \times 10^{-2} \mathrm{M}\). The value of x is ______. (Nearest integer)

JEE Main · 2021 · 25 Jul 2021 · Shift 1 · Redox Reactions and Electrochemistry

When 10 ml of an aqueous solution of

Fe^{2 +}

ions was titrated in the presence of dil

H_{2} SO_{4}

using diphenylamine indicator, 15 mL of 0.02 M solution of

K_{2} Cr_{2} O_{7}

was required to get the end point. The molarity of the solution containing

Fe^{2 +}

ions is

x \times 1 0^{- 2} M

. The value of x is ______. (Nearest integer)

Answer: 18

In acid solution dichromate oxidises iron(II) according to

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

so six iron(II) ions are consumed for every dichromate ion. The titrant supplies

0.02 \times \frac{15}{1000} = 3 \times 1 0^{- 4}

mol of dichromate, which oxidises

6 \times 3 \times 1 0^{- 4} = 1.8 \times 1 0^{- 3}

mol of iron(II). That amount is contained in 10 mL, so the concentration is

\frac{1.8 \times 1 0 ^{- 3}}{10 \times 1 0 ^{- 3}} = 0.18 M = 18 \times 1 0^{- 2} M

. The value asked for is

18

JEE Main · 2021 · 25 Jul 2021 · Shift 1 · Integer answer