Question

Correct statements from the following are:
A. Potassium dichromate is an oxidising agent and it oxidises FeSO₄ to Fe₂(SO₄)₃ in acidic medium.
B. Sodium dichromate can be used as primary standard in volumetric estimation.
C. CrO₄²⁻ and Cr₂O₇²⁻ are interconvertible in aqueous solution by varying the pH of the solution.
D. Cr-O-Cr bond angle in Cr₂O₇²⁻ is 126°.
Choose the correct answer:

• A. A, B and C Only
• B. A, C and D Only
• C. A and C Only
• D. B and D Only

Answer 1

The Correct Option is B

Step 1: Understanding the Concept:
Dichromates are powerful oxidizing agents in acidic media. Their chemical properties involve pH-dependent equilibria and specific structural parameters in the solid state.

Step 2: Key Formula or Approach:
1. Reaction: $Cr_2O_7^{2-} + 14H^+ + 6Fe^{2+} \to 2Cr^{3+} + 6Fe^{3+} + 7H_2O$. 2. Equilibrium: $2CrO_4^{2-} + 2H^+ \rightleftharpoons Cr_2O_7^{2-} + H_2O$.

Step 3: Detailed Explanation:
1. Statement A analysis: $K_2Cr_2O_7$ is a well-known oxidant in acidic medium and converts $Fe^{2+}$ (ferrous) to $Fe^{3+}$ (ferric). This is true. 2. Statement B analysis: Sodium dichromate ($Na_2Cr_2O_7$) is hygroscopic (absorbs moisture), which prevents it from being a primary standard. Potassium dichromate is used instead because it is non-hygroscopic. This is false. 3. Statement C analysis: Chromate ($CrO_4^{2-}$, yellow) and dichromate ($Cr_2O_7^{2-}$, orange) exist in equilibrium. Low pH (acidic) favors dichromate; high pH (basic) favors chromate. This is true. 4. Statement D analysis: The dichromate ion consists of two $CrO_4$ tetrahedra sharing an oxygen atom, with a $Cr-O-Cr$ bond angle of 126°. This is true.

Step 4: Final Answer:
Statements A, C, and D are correct.