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:
$4c{l}^{-}\left(s\right)+c{r}_2{o}_7^{2-}\left(s\right)+6H^{+}\left(conc.\right)\to 2Cr{O}_{2}c{l}_2\left(deepredvapours\right)+3H_{2}O$
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.
$Cr{O}_{2}C{l}_2+4O{H}^{-}\to Cr{O}_4^{2-}+2c{l}^{-}+2H_{2}O$
$Cr{O}_4^{2-}+P{b}^{+2}\to PbCr{O}_4\downarrow \left(yellow\right)$