Question

During the preparation of \(\mathrm{K_2Cr_2O_7}\) from chromite ore, in one of the steps, the yellow solution of sodium chromate is converted into orange sodium dichromate crystals. This is achieved by

• A. Increasing the pH
• B. Decreasing the pH
• C. Maintaining neutral pH
• D. Adding NaCl \bigskip

Hint:

Remember the chromate--dichromate equilibrium: \(\displaystyle 2\mathrm{CrO_4^{2-}}+2\mathrm{H^+}\rightleftharpoons\mathrm{Cr_2O_7^{2-}}+\mathrm{H_2O}\).

• Basic medium \(\rightarrow\) Yellow chromate (\(\mathrm{CrO_4^{2-}}\)).
• Acidic medium \(\rightarrow\) Orange dichromate (\(\mathrm{Cr_2O_7^{2-}}\)).

Answer 1

The Correct Option is B

Step 1: Yellow Na₂CrO₄ (CrO₄²⁻) + acid ⇌ orange Na₂Cr₂O₇ (Cr₂O₇²⁻) + H₂O. Decreasing pH shifts equilibrium to dichromate.

Step 2: Write the final answer. \(\boxed{\text{decreasing the pH}.}\)