Question

One mole of ammonia was completely absorbed in one litre solution each of
(I) $1M \,HCl$,
(II) $1M \,CH_3COOH$ and
(III) $1M \,H_2SO_4$ at $298\, K$.
The decreasing order for the pH of the resulting solutions
(Given $ K_b(NH_3)$ = 4.74)

• A. I > II > III
• B. III > II >I
• C. II > III >I
• D. II > I > III

Answer 1

Answer: (D)

Acids which are weak in water will be highly acidic in $NH _{3}$

(a) $HCl + NH _{3} \rightleftharpoons NH _{4} Cl$

(b) $CH _{3} COOH + NH _{3} \rightleftharpoons CH _{3} COONH _{4}$

(c) $H _{2} SO _{4}+ NH _{3} \rightleftharpoons\left( NH _{4}\right)_{2} SO _{4}$

$\therefore $ Effect of ammonia is least in $H _{2} SO _{4}$ and maximum in $CH _{3} COOH$.

$\therefore $ Order of acidic strength of resulting solution is

$\left( NH _{4}\right)_{2} SO _{4}> NH _{4} Cl > CH _{3} COONH _{4}$

$\therefore $ Order of $pH$ of resulting solution is

$CH _{3} COONH _{4}> NH _{4} Cl >\left( NH _{4}\right)_{2} SO _{4}$

Hence, correct option is II $> I > III$