Question

Presence of ${Fe}^{3+}$ in an aqueous solution can be made by addition of sodium acetate (in excess) when a reddish-brown colour is formed.

Excess of sodium acetate is added in the above test. It is because__________.

• A. It acts as a buffer to control pH of the reaction otherwise reaction is made reversible
• B. It makes the solution alkaline which destroys acidic effect if any
• C. Sodium acetate has low $K_{sp}$ value
• D. Sodium acetate has high $K_{sp}$ value

Answer 1

The correct option is A It acts as a buffer to control pH of the reaction otherwise reaction is made reversible

In aqueous medium, Fe(III) ions react with acetic acid to form reddishbrown complex, $\left[{Fe}_3\right(OH{)}_2({CH}_{3}COO{)}_6{]}^{+}$

$3F{e}^{3+}+2H_{2}O+6C{H}_{3}COONa\rightleftharpoons \left[{Fe}_3\right(OH{)}_2\left({CH}_{3}COO{)}_6{]}^{+}\right(reddishbrown)+2H^{+}+6N{a}^{+}$

If hydrogen ions are present in excess, then the reaction will shift in backward direction. To avoid this, sodium acetate is taken in excess which will act as buffer and control pH.

Therefore, option A is correct.