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