Question

\[C{{r}_{2}}O_{7}^{2-}\left( orange \right)\underset{pH=Y}{\overset{pH=X}{\longleftrightarrow}}CrO_{4}^{2-}\left( green \right)\]
This change is based on the change in pH. What is the probable value of X and Y?
(A) 8,6
(B) 8,10
(C) 4,6
(D) change is independent of pH

Answer 1

Hint: Chromate exists in alkaline pH whereas dichromate exists in acidic pH. The numerical value of X will always be greater than Y.

Complete step by step solution:
Let us get familiar with the ions we are going to work with. As it is very clear from the molecular formula, the chromate ion contains one chromium molecule and the dichromate ion contains two chromium molecules. But when we calculate the oxidation state of the chromium atoms in both of the ions, it turns out that they have the same numbers. In both of the ions, the oxidation state of chromium is +6. It is the highest oxidation state of chromium and therefore cannot further oxidise, which makes it a good oxidising agent.
It has been found experimentally, that chromate and dichromate ions are interconvertible in a solution and that they maintain an equilibrium between them. The equilibrium, to a much extent, is pH-dependent. The chromate ions exist in alkaline pH whereas the dichromate ions exist in acidic pH. The reaction that takes place when the interconvert is as shown below:
\[2CrO_{4}^{2-}+2{{H}^{+}}\rightleftharpoons C{{r}_{2}}O_{7}^{2-}+{{H}_{2}}O\]
As you can see, the chromate ion converts into dichromate when the concentration of protons or hydronium ions increases in the solution or it can also be said that this change occurs when the pH of the solution decreases.

Out of all the options given above, option(A) has X as 8 (basic) and Y as 6 (acidic). So, this is our answer.

Note: All the other options either had pH values that were both acidic or basic. Even if we don’t know which one of the ions predominates in what pH, we could still have guessed our answer based on the knowledge that one of them exists in acidic condition whereas the other in basic condition.