Question

A mixture of NaCl and K₂Cr₂O₇ is heated with conc. H₂SO₄, deep red vapours and formed. Which of the following statement is false?

• A. The vapours contain CrO₂Cl₂ and Cl₂.
• B. The vapours when passed into lead acetate in acetic acid gives a yellow precipitate.
• C. The vapours give a yellow solution with NaOH.
• D. The vapours contain CrO₂Cl₂ only.

Answer 1

Answer: (A)

Chromyl chloride test:
$4cl^{-}\left(s\right)+cr_{2}o_{7}^{2-}\left(s\right)+6H^{+}\left(conc.\right) \to2CrO_{2}cl_{2} \left(deep\, red \,vapours\right)+3H_{2}O$
When deep red vapours are passed into sodium hydroxide solution, a yellow solution of sodium chromate is formed, which when treated with lead acetate gives yellow precipitate of lead chromate.
$CrO_{2}Cl_{2}+4OH^{-} \to CrO_{4}^{2-}+2cl^{-}+2H_{2}O$
$CrO_{4}^{2-}+Pb^{+2}\to PbCrO_{4} ↓ \left(yellow\right)$