JEE Main · 2020 · 05 Sep 2020 · Shift 2 · Redox Reactions and Electrochemistry

The volume, in mL , of

0.02 M K_{2} Cr_{2} O_{7}

solution required to react with 0.288 g of ferrous oxalate in acidic medium is ____ . (Molar mass of

Fe = 56 mol^{- 1}

)

Answer: 50

Solution

In acid, dichromate is reduced

Cr X^{6 +} Cr X^{3 +}

, with two chromium atoms each gaining three electrons, so

K X_{2} Cr X_{2} O X_{7}

has an

n

-factor of

6

. In ferrous oxalate both centres are oxidised,

Fe X^{2 +} Fe X^{3 +}

(one electron) and

C X_{2} O X_{4} X^{2 -} 2 CO X_{2}

(two electrons), giving an

n

-factor of

3

. With molar mass

144

g/mol, the

0.288

g sample provides

\frac{0.288}{144} \times 3 = 0.006

equivalents. Equating equivalents,

0.02 \times 6 \times V (L) = 0.006

, so

V = 0.05

= 50

mL.

JEE Main · 2020 · 05 Sep 2020 · Shift 2 · Integer answer