Question

In the third group of qualitative analysis, the precipatitating reagent is $N{H}_{4}Cl/N{H}_{4}OH$. The function of $N{H}_{4}Cl$ is to

• A. increase the ionisation of $N{H}_{4}OH$
• B. supress the ionisation of $N{H}_{4}OH$
• C. convert the ions of group third into their respective chlorides
• D. stabilise the hydroxides of group $III$ cations

Answer 1

Answer: (B)

supress the ionisation of $N{H}_{4}OH$

Adding a common ion prevents the weak acid or weak base from ionizing as much as it would without the added common ion. The common ion effect suppresses the ionization of a weak acid by adding more of an ion that is a product of this equilibrium.

So, addition of $C{H}_{3}COONa$ to $C{H}_{3}COOH$ increases the pH of solution but addition of $N{H}_{4}Cl$ to $N{H}_{4}OH$ decreases the pH of solution.

So, the function of $N{H}_{4}Cl$ to suppress the ionisation of $N{H}_{4}OH$ and to reduce conentration of hydroxide ion in solution.