Question

Solid $KCl$, when heated with solid $K_{2}C{r}_2{O}_7$ and concentrated $H_{2}S{O}_4$, gives red vapours (a) that turn $NaOH$ solution yellow (b). The yellow solution, when acidified with acetic acid and treated with lead acetate, gives a yellow precipitate (c). Which of the following is true with respect to a, b and c?

• A. a and b contain $Cr{O}_4^{2-}$
• B. a and c contain $Cr{O}_4^{2-}$
• C. a, b and c contain $Cr{O}_4^{2-}$
• D. a, b and c contain $Cr\left(VI\right)$

Answer 1

The correct option is D a, b and c contain $Cr\left(VI\right)$

Potassium dichromate react with potassium chloride and sulfuric acid to produce chromyl chloride $Cr{O}_{2}C{l}_2$ which forms red vapours. In the compound $Cr$ is in $+6$ oxidation state.

$K_{2}C{r}_2{O}_7+4KCl+3H_{2}S{O}_4\to 2Cr{O}_{2}C{l}_2+3K_{2}S{O}_4+3H_{2}O$

This turns $NaOH$ solution yellow by formation of sodium chromate, $N{a}_{2}Cr{O}_4\Rightarrow Cr$ is in $+6$ oxidation state.

$Cr{O}_{2}C{l}_2+4NaOH\to N{a}_{2}Cr{O}_4+2NaCl+2H_{2}O$.

This when acidified with acetic acid and treated with lead acetate, gives a yellow precipitate. The yellow precipitate is Lead chromate. Its formula is $PbCr{O}_4\Rightarrow Cr$ is in $+6$ oxidation state.

$N{a}_{2}Cr{O}_4+Pb(C{H}_{3}COO{)}_2\to PbCr{O}_4+C{H}_{3}COONa$