The pKa value of is 5. Calculate the pH of the buffer solution, 1 L of which contains 0.01 M and 0.1 — Ionic Equilibrium Chemistry Question

💡 Solution & Explanation

# Solution: pH of Ammonia Buffer **Step 1: Identify the buffer system** This is a basic buffer containing: - Weak base: $NH_4OH$ (0.10 M) — the conjugate base - Conjugate acid: $NH_4Cl$ (0.01 M) — provides $NH_4^+$ ions **Step 2: Clarify the pKa value** The given pKa = 5 refers to $NH_4^+$ (the conjugate acid): $$NH_4^+ \rightleftharpoons NH_3 + H^+, \quad pK_a = 5$$ Therefore: $pK_b$ of $NH_3$ = 14 − 5 = **9** **Step 3: Apply Henderson-Hasselbalch equation** For the base form: $$pOH = pK_b + \log\frac{[NH_4^+]}{[NH_3]}$$ $$pOH = 9 + \log\frac{0.01}{0.10}$$ $$pOH = 9 + \log(0.1) = 9 + (-1) = 8$$ **Step 4: Calculate pH** $$pH = 14 - pOH = 14 - 8 = \boxed{6}$$ **Why this is correct:** The buffer equation uses the ratio of acid to base in the logarithm. Since we have more base ($NH_4OH$) than acid ($NH_4Cl$), the solution is slightly basic with pH = 6, which is reasonable for an ammonia buffer system.