A 10 g sample of CuS and Cu2S was treated with 100 mL of 1.25 M K2Cr2O7 to produce Cr3+, Cu2+ and SO2. The excess oxidant was reacted with 50 mL of Fe2+ solution. 25 mL of the same Fe2+ solution required 0.875 M KMnO4 under acidic condition, the volume of KMnO4 used was 20 mL. Find the percentage of Cu2S in the sample.
Given: molar mass of Cu is 63.5 g mol−1