Consider a titration of potassium dichromate solution with acidified Mohr's salt solution using diphenylamine as indicator. The number of moles of Mohr's salt required per mole of dichromate is:

Options

A. 3
B. 4
C. 5
D. 6

Answer

D

Explanation

`overset(+6)(Cr_(2))O_(7)^(2-)rarrCr^(3+)` (Reduction)
`Cr_(2)O_(7)^(2-)+14H^(+)+6e^(-)rarr2Cr^(3+)+7H_(2)O....(i)`
`Fe^(2+)rarrFe^(3+)` (Oxidation)
(Mohr.s salt is `FeSO_(4).(NH_(4))_(2)SO_(4).6H_(2)O` where iron is in `Fe^(2+)` state)
`6xx[Fe^(2+)rarrFe^(3+)+e^(-)]......(ii)`
Adding eq. (i) and (ii):
`Cr_(2)O_(7)^(2-)+6Fe^(2+)+14H^(+)rarr2Cr^(3+)+7H_(2)O+6Fe^(3+)`
Hence 6 moles of mohr.s salt are required per mole of dichromate ion.