Question

Determine the effect on the concentrations of NH₃, NH₄⁺ and OH^– when small amounts of each of the following is added to a solution of NH₃ in water.

(A) HCl 

(B) NH₃ 

(C) NaOH 

(D) NH₄Cl 

(E) KNO₃

Answer 1

The equilibria of NH₃ in water will be

NH₃ + H₂O ⇌ NH₄⁺ (aq.) + OH^– (aq.)

(A) If HCl is added it will furnish H⁺ and Cl^– ions as it is a strong electrolyte. Now H⁺ will form H₂O with OH^– ions (neutralisation) so [NH₃] will decrease, [NH₄⁺] and [OH^–] increase since equilibria will shift in forward direction. 

(B) If more NH₃ is added, then [NH₃], [NH₄⁺] and [OH^–] will increase. 

(C) If NaOH is added it will furnish [Na⁺] and [OH^–] ions since it is a strong electrolyte and [OH^–] will suppress the ionisation of weak base NH₃ so [NH₃] increases, [NH₄⁺] decreases and [OH^–] increases. 

(D) NH₄Cl will also apply common ion effect on NH₄OH but now [NH₃] increases, [NH₄⁺] increases but [OH^–] decreases. 

(E) NO effect expected