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 Kb = 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

Answer:

pH = 9.6

Explanation:

According to Brönsted-Lowry theory, NH₃ is a base and NH₄⁺ its conjugate acid. When they are together in a solution, the form a buffer, which is used to resist abrupt changes in pH when an acid or a base is added. pOH fro a buffer can be found using Henderson-Hasselbalch equation.

$pOH = pKb + log\frac{conjugateacid}{base}$

Since NH₄Cl is a strong electrolyte, [NH₄Cl] = [NH₄⁺]

$pOH = pKb + log\frac{[NH_{4}^{+} ]}{[NH_{3}]} =4.7+log\frac{0.035M}{0.070M} =4.4$

Now, we can find pH using the following expression:

pH + pOH = 14

pH = 14 - pOH = 14 - 4.4 = 9.6