Question

Consider the given reaction equation
\[C{{r}_{2}}O_{7}^{2-}\xrightarrow{pH=x}CrO_{4}^{2-}\xrightarrow{pH=y}C{{r}_{2}}O_{7}^{2-}\]
The pH values of x and y can be:
A. 4 and 5
B. 4 and 8
C. 8 and 4
D. 8 and 9

Answer 1

Hint: The pH of the solution will determine the direction in which reaction will proceed. According to Le-chatelier's principle if the amount or concentration of reactant is increased then the reaction will move forwards and if the amount or concentration of product is increased then the reaction will move backwards.

Complete step by step answer:
The equilibrium reaction of conversion of chromate ion to dichromate ion is given below:
\[2CrO_{4}^{2-}+2{{H}^{+}}\rightleftharpoons C{{r}_{2}}O_{7}^{2-}+{{H}_{2}}O\]

The conversion of the dichromate ion to the chromate ion usually takes place in an alkaline medium which means that for this reaction to proceed in forward direction a solution with pH greater than 7 is required.
The conversion of the chromate ion again to the dichromate ion has to take place in an acidic medium which means that for this reaction to proceed in forward direction a solution with pH less than 7 is required.
\[C{{r}_{2}}O_{7}^{2-}\xrightarrow{pH=x>7}CrO_{4}^{2-}\xrightarrow{pH=y<7}C{{r}_{2}}O_{7}^{2-}\]
Hence, the correct answer is that pH values x and y can be 8 and 4.
So, the correct answer is “Option C”.

Note: When the concentration of hydrogen ion is more in the solution, that it is acidic in nature, we then call it acidic medium. When the concentration of hydroxide ion is more in the solution that is it is basic in nature, we then call it basic or alkaline medium.