The number of moles of potassium dichromate needed to oxidize 3 moles of ferrous ions in acidic medium will be ………..

0.5

The correct answer is Option (1) → 0.5

Potassium dichromate is used as a primary standard in volumetric analysis. In acidic solution, its oxidizing action can be represented as follows:

$ \text{Cr}_2\text{O}_7^{2-}+14\text{H}^{+}+6\text{e}^{-}\rightarrow2\text{Cr}^{3+}+7\text{H}_2\text{O}\quad(E=1.33\text{V}) $

Thus, acidified potassium dichromate will oxidize iron (II) salts to iron (III). The half-reaction is noted below

$ 6\text{Fe}^{2+}\rightarrow6\text{Fe}^{3+}+6\text{e}^{-} $

The full ionic equation may be obtained by adding the half-reaction for potassium dichromate to the half-reaction for the reducing agent,

$ \text{Cr}_2\text{O}_7^{2-}+14\text{H}^{+}+6\text{Fe}^{2+}\rightarrow2\text{Cr}^{3+}+6\text{Fe}^{3+}+7\text{H}_2\text{O} $

1 mole of dichromate ion oxidizes 6 moles of $Fe^{2+}$

Therefore, to oxidize 3 moles

Moles of $ \text{K}_2\text{Cr}_2\text{O}_7=0.5\text{ moles} $