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

To determine the number of moles of Mohr's salt required per mole of potassium dichromate in the titration, we can follow these steps:

### Step-by-Step Solution:

1. **Identify the Components**:
- Potassium dichromate (K₂Cr₂O₇) is the oxidizing agent.
- Mohr's salt (FeSO₄·(NH₄)₂SO₄·6H₂O) contains ferrous sulfate (FeSO₄), which acts as the reducing agent.

2. **Write the Reaction**:
- In an acidic medium, the reaction between potassium dichromate and ferrous sulfate can be represented as:
\[ K_2Cr_2O_7 + 6FeSO_4 + 14H_2SO_4 \rightarrow 2Cr_2(SO_4)_3 + 6Fe_2(SO_4)_3 + 7H_2O \]
- In this reaction, potassium dichromate (Cr in +6 oxidation state) is reduced to chromium sulfate (Cr in +3 oxidation state), while ferrous sulfate (Fe in +2 oxidation state) is oxidized to ferric sulfate (Fe in +3 oxidation state).

3. **Determine Stoichiometry**:
- From the balanced equation, we can see that 1 mole of potassium dichromate reacts with 6 moles of ferrous sulfate.
- Therefore, the stoichiometric ratio is:
\[ 1 \text{ mole of } K_2Cr_2O_7 : 6 \text{ moles of } FeSO_4 \]

4. **Calculate Moles of Mohr's Salt**:
- Since Mohr's salt contains 1 mole of ferrous sulfate per mole of Mohr's salt, the number of moles of Mohr's salt required will also be 6 for every mole of potassium dichromate.
- Thus, the number of moles of Mohr's salt required per mole of potassium dichromate is:
\[ 6 \text{ moles of Mohr's salt} \]

### Final Answer:
The number of moles of Mohr's salt required per mole of potassium dichromate is **6 moles**.