A mixture of acidified `K_(2)Cr_(2)O_(7)` and 10% Kl is titrated against against `Na_(2)S_(2)O_(3)` (sodium thiosulphate) solution using starch indicator. The colour of the reaction m ixture at the end point is:

Options

A. Yellow
B. Blue
C. Green
D. Colourless

Answer

To solve the problem, we will analyze the titration process involving acidified potassium dichromate (K₂Cr₂O₇), potassium iodide (KI), and sodium thiosulfate (Na₂S₂O₃) using starch as an indicator.

### Step-by-Step Solution:

1. **Understanding the Components**:
- **K₂Cr₂O₇**: This is potassium dichromate, which is a strong oxidizing agent in acidic medium.
- **KI**: Potassium iodide provides iodide ions (I⁻), which can be oxidized by K₂Cr₂O₇.
- **Na₂S₂O₃**: Sodium thiosulfate is used as a reducing agent in this titration.
- **Starch Indicator**: Starch forms a blue complex with iodine (I₂), which helps to indicate the endpoint of the titration.

2. **Reaction Overview**:
- In the presence of acid, K₂Cr₂O₇ oxidizes iodide ions (I⁻) from KI to iodine (I₂).
- The reaction can be represented as:
\[ K_2Cr_2O_7 + 6KI + 14H^+ \rightarrow 2Cr^{3+} + 3I_2 + 7H_2O + 2K^+ \]
- The iodine produced (I₂) is then titrated with Na₂S₂O₃, which reduces iodine back to iodide ions:
\[ I_2 + 2Na_2S_2O_3 \rightarrow 2NaI + Na_2S_4O_6 \]

3. **Identifying the Endpoint**:
- Initially, the solution will have a brown color due to the presence of iodine (I₂).
- As Na₂S₂O₃ is added, it reduces the iodine, and the brown color will fade.
- The endpoint is reached when all iodine has reacted, and a slight excess of Na₂S₂O₃ will cause the solution to turn from brown to colorless.
- However, upon the addition of starch, if there is still a small amount of iodine present, it will form a blue complex with starch.

4. **Final Color at the Endpoint**:
- At the endpoint, when all iodine has been reduced and there is no excess iodine, the solution will appear green due to the presence of chromium ions (Cr³⁺) and the absence of iodine.
- Therefore, the color of the reaction mixture at the endpoint is **green**.

### Conclusion:
The color of the reaction mixture at the endpoint of the titration is **green**.