A 10 g samle of only CuS and Cu_(2)S was treated with 100 ml of 1.25 M K_(2) Cr_(2) O_(7).The Products obtained were Cr^(3+) , Cu^(2+) " and " SO_(2) . The excess oxidant was reacted with 50 ml of Fe^(2+) Solution : 25 ml of the same Fe^(2+) solution required 0.875 M KMnO_(4) under acidic condition , the volume of KMnO_(4) used was 20 ml . Find the % of CuS and Cu_(2)S in the sample .

A 10 g samle of only CuS and Cu_(2)S was treated with 100 ml of 1.25 M

1.	A 10 g samle of only CuS and Cu_(2)S was treated with 100 ml of 1.25 M K_(2) Cr_(2) O_(7).The Products obtained were Cr^(3+) , Cu^(2+) " and " SO_(2) . The excess oxidant was reacted with 50 ml of Fe^(2+) Solution : 25 ml of the same Fe^(2+) solution required 0.875 M KMnO_(4) under acidic condition , the volume of KMnO_(4) used was 20 ml . Find the % of CuS and Cu_(2)S in the sample .
Answer» Solution :EQUIVALENTS of DICHROMATE initially` = (1.25 xx 6 xx 100)/1000 = 0.75` Equivalents of `Fe^(2+) ` in 25 ml` = (0.875 xx 5 xx 5 xx 20)/1000 = 0.0875` Equivalents ofexcess dichromate = `0.175` ` :. ` Equivalents of dichromate consumed by `(CuS" and " Cu_(2)S)` ` = 0.75 - 0.175 = 0.575` Ifx G is the mass of CuS, the mass of `Cu_(2)S " is " (10 - x) ` g `x/(95.5) xx 6 + ((10 -x))/(159) xx 8 = 0.575` ` :. x = 5.74 ` gm ` %CuS = (5.74)/10 xx 100 = 57.4 %` ` % Cu_(2)S = 42.6 %`