Some Basic Concepts of Chemistry Part 1

Cr2O72−

+6

+

Fe2+

+2

⟶

Cr3+

+3

+

Fe3+

+3

Dichromate (Cr2O72−) oxidised Fe2+ to Fe3+ and itself get reduced to Cr3+.
Valency factor of Cr2O72−=6 as Cr gets reduced from +6 to +3
From law of equivalence,
Milliequivalent of Cr2O72−= Milliequivalent of Fe2+
M1V1n1=M2V2n2
(M1×V1)×6=(M2×V2)×1
(0.03×20)×6=(M2×15)1
⇒‌‌‌‌M2=0.24 molar
=24×10−2 molar