Question

$\underset{orange}{C{r}_2{O}_7^{2-}}\stackrel{pH=x}{\underset{pH=y}{\leftrightarrows }}\underset{green}{Cr{O}_4^{2-}}$.

This change is based on change in pH.

Find values of x and y :

• A. $6,8$
• B. $8,10$
• C. $4,6$
• D. change is independent of pH

Answer 1

Answer: (A)

$6,8$

The process involves the equilibrium between chromate (VI), dichromate (VI) and hydrogen ions:

$2{Cr{O}_4^{2-}}_{(aq.)\left(yellow\right)}+2{H^{+}}_{(aq.)}\rightleftharpoons {{Cr}_2{O}_7^{2-}}_{(aq.)\left(orange\right)}+{H_{2}O}_{\left(l\right)}$

The addition of acid encourages the equilibrium towards the right (pH=6), producing more orange colored dichromate (VI) ions. The addition of hydroxide ions causes the concentration of hydrogen ions to decrease, and this brings the equilibrium back to the left-hand side, regenerating yellow chromate (VI) ions.