Question
An orange coloured solution acidified with $${H_2}S{O_4}$$ and treated with a substance $$'X'$$ gives a blue coloured solution of $$Cr{O_5}.$$ The substance $$'X'$$ is
A.
$${H_2}{O_2}$$
B.
$${H_2}O$$
C.
$$dil\,HCl$$
D.
$$conc\,HCl$$
Answer :
$${H_2}{O_2}$$
Solution :
$${K_2}C{r_2}{O_7}$$ an orange coloured solution, on acidification with $$dil\,{H_2}S{O_4}$$ is oxidised by $${H_2}{O_2}$$ to blue peroxide of chromium, $$Cr{O_5},$$ which is unstable.
$$\eqalign{
& {K_2}C{r_2}{O_7} + {H_2}S{O_4} \to {K_2}S{O_4} + {H_2}C{r_2}{O_7} \cr
& \left[ {{H_2}{O_2} \to {H_2}O + \left[ O \right]} \right] \times 4 \cr
& \underline {{H_2}C{r_2}{O_7} + 4\left[ O \right] \to 2Cr{O_5} + {H_2}O\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,} \cr
& \underline {{K_2}C{r_2}{O_7} + {H_2}S{O_4} + 4{H_2}{O_2} \to 2Cr{O_5} + {K_2}S{O_4} + 5{H_2}O} \cr} $$