The correct statement is

Options

A. Ptassium dichromate is more soluble than sodium dichromate
B. All `Cr-O` bond lengths in dichromate ion are equal.
C. Potassium dichromate is used as a primary standard in volumetric titrations
D. All are correct

Answer

To solve the question regarding which statement about potassium dichromate and sodium dichromate is correct, we will analyze each statement one by one.

### Step-by-Step Solution:

1. **Analyzing Solubility of Potassium Dichromate and Sodium Dichromate**:
- Potassium dichromate has the formula \( K_2Cr_2O_7 \) and sodium dichromate has the formula \( Na_2Cr_2O_7 \).
- The cations present are \( K^+ \) for potassium dichromate and \( Na^+ \) for sodium dichromate.
- Sodium ion (\( Na^+ \)) is smaller in size compared to potassium ion (\( K^+ \)) and has higher hydration energy.
- Therefore, sodium dichromate is more soluble than potassium dichromate.
- **Conclusion**: This statement is **incorrect**.

2. **Analyzing Bond Lengths in Dichromate Ion**:
- The dichromate ion has the formula \( Cr_2O_7^{2-} \).
- In the structure of dichromate, there are resonance structures that involve the oxygen atoms.
- There are 6 \( Cr-O \) bonds that are equal due to resonance, but the 2 \( Cr-O \) bonds that are not involved in resonance are different.
- Therefore, not all \( Cr-O \) bond lengths are equal.
- **Conclusion**: This statement is **incorrect**.

3. **Analyzing Potassium Dichromate as a Primary Standard**:
- A primary standard is a reagent that is pure, stable, and has a known concentration.
- Potassium dichromate is known for its high purity and stability in air, making it suitable for use as a primary standard in volumetric titrations.
- **Conclusion**: This statement is **correct**.

4. **Final Statement Analysis**:
- The last statement claims that "all are correct." Since we have established that the first two statements are incorrect and only the third statement is correct, this overall statement is also incorrect.

### Final Conclusion:
The only correct statement is: **Potassium dichromate is used as a primary standard in volumetric titrations.**

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