what is the pH of the solution?

Question

Ammonia (NH₃) ionizes according to the following reaction:

NH₃(aq) + H₂O(l) ⇌ NH₄⁺(aq) + OH^–(aq)

The base dissociation constant for ammonia (NH₃) is K_b = 1.8 × 10^–5. Ammonia (NH₃) also has a chloride salt, ammonium chloride (NH₄Cl), which is soluble in water.

If 0.070 M of ammonia (NH₃) and 0.035 M of its salt ammonium chloride (NH₄Cl) are mixed in a solution, what is the pH of this solution?

Answer 1

The mixture of NH₃ and NH₄Cl represents a buffer system as it contains a weak base (NH₃) and the conjugate acid (NH₄). We can use the Henderson Hasselbalch equation:

pOH = pKb + log [NH₄] / [NH₃] and pKb = -log Kb = 4.75

pOH = 4.75 + log (0.035/0.070) = 4.75 + (-0.30)

pOH = 4.45

pH = 14 - pOH = 14 - 4.45

pH = 9.55