20 mL of a solution containing ferrous sulphate and ferric sulphate acidified with `H_(2)SO_(4)` is reduced by metallic zinc. The solution required 27.4 mL of 0.1 N solution of `K_(2)Cr_(2)O_(7)` for oxidation. However before reduction with zinc, 20 mL of same solution required 17.96 mL of same `K_(2)Cr_(2)O_(7)`. Calculate the mass of `FeSO_(4) " and " Fe_(2)(SO_(4))_(3)` per litre of the solution.

Explanation

After reduction `Fe_(2)(SO_(4))_(3)` is also reduced to `FeSO_(4)` and titration gives total concentration of `FeSO_(4) " and" Fe_(2)(SO_(4))_(3)`.
Titration before reduction gives only `FeSO_(4)`.
Milli equiv. of `K_(2)Cr_(2)O_(7)` after reduction `=27.4xx0.1=2.740`
Milli equiv. of `K_(2)Cr_(2)O_(7)` before reduction `=17.96xx0.1=1.796`
Milli equiv. of `Fe_(2)(SO_(4))_(3)` in 20 mL =0.944
Milli equiv. of `FeSO_(4)` in 20 mL =1.796
`therefore FeSO_(4)(gL^(-1))=(1.796)/(1000)xx"Eq. mass"xx40`
`=(1.796)/(1000)xx152xx40=10.92`
`Fe_(2)(SO_(4))_(3)(gL^(-1))=(0.944)/(1000)xx"Eq. mass"xx40`
`=(0.944)/(1000)xx200xx40=7.55`