Question

We have to find the value of x and y ,$CrO_4^{2 - }\xrightarrow{{pH = x}}C{r_2}O_7^{2 - }\xrightarrow{{pH = y}}CrO_4^{2 - }$ so x and y can be:
A.3 and 5
B.3 and 8
C.8 and 3
D.8 and 11

Answer 1

Hint: The chromate ion is present in equilibrium with the dichromate ion within a certain pH. When this pH changes these ions are inter converts.

Complete step by step answer:
We know that the colour of dichromate ion that is $C{r_2}O_7^{2 - }$ is orange red and that of chromate ion ($Cr{O_4}^{2 - }$) is yellow in colour. When chromate ion is added to an acidic solution it gets converted to dichromate ion. Here acidic solution means that the pH of the solution must be less than 7. Therefore when chromate ion is acidified the yellow colour of chromate ion changes into red orange colour of dichromate ion. Also when an alkali that is basic in nature is added to the red orange solution of dichromate ion , a yellow solution appears due to the formation of chromate ions. Here basic solution means that the pH of the solution must be greater than 7.
Therefore for the given reaction as we talk earlier than when an acidic solution is added chromate ion gets converted into dichromate ion , therefore here the value of x must be smaller than 7. Also when an alkali is added to dichromate ion, it is again converted to chromate ion ,hence the pH must be greater than 7. Here in the given option (B) option satisfying the condition. Hence $x = 3{\text{ and y}} = {\text{8}}$.
$CrO_4^{2 - }\xrightarrow{{pH = 3}}C{r_2}O_7^{2 - }\xrightarrow{{pH = 8}}CrO_4^{2 - }$
Hence the correct option is B.

Note:
It should be noted that the oxidation state of chromium in chromate ($Cr{O_4}^{2 - }$) and dichromate($C{r_2}O_7^{2 - }$) is same ,i.e 6.