When sodium chromate is acidified with sulfuric acid and then further treated with potassium chloride, we obtain....
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → Potassium dichromate
Chromate and dichromate ions change into each other with the pH of the solution; chromium stays in the +6 state in both. In acid, yellow chromate changes to orange dichromate:
\( \displaystyle \ce{2CrO4^2- + 2H+ -> Cr2O7^2- + H2O} \)
So acidifying sodium chromate with sulphuric acid gives sodium dichromate:
\( \displaystyle \ce{2Na2CrO4 + H2SO4 -> Na2Cr2O7 + Na2SO4 + H2O} \)
Potassium chloride then exchanges the cation, so the final product is potassium dichromate.
Why the other options are incorrect
Option (1) Sodium dichromate — Incorrect. This is the intermediate formed on acidification; potassium chloride then converts it into the potassium salt.
Option (2) Potassium chromate — Incorrect. In acidic solution the chromium is present as dichromate, not chromate; chromate is formed back only in alkaline solution.
Option (4) Sodium dichromate dihydrate — Incorrect. \( \ce{Na2Cr2O7.2H2O} \) is what crystallises after acidification alone, before potassium chloride is added.
NCERT (2026-27 edition):
Potassium Dichromate K2Cr2O7 – …
