There are two broad aspects of this chapter:
Acid/ Base Equilibria
Important Relations
At 25°C:
Approach:
Determine what is present in solution: Acid/ Base/ Salt/ Buffer
Apply the corresponding equations:
Acid/ Base
Formula | Example | |
Strong Acid | [H+] = [HA] pH = -log [HA] | pH of 0.100 moldm–3 HCl
pH = -log [0.100] = 1.0 |
Strong Base | [OH–] = [B] pOH = -log [B] | pH of 0.100 moldm–3 NaOH
pOH = -log [0.100] = 1.0 pH = 14.0 - 1.0 = 13.0 |
Weak Acid | pH = -log [H+] | pH of 0.100 moldm–3 CH3COOH (Ka = 1.8 x 10–5 moldm–3)
[H+] = [(1.8 x 10–5)(0.100)]1/2 = 1.34 x 10–3 mol dm–3 pH = -log [1.34 x 10–3] = 2.9 |
Weak Base | pOH = -log [OH–] | pH of 0.100 moldm–3 NH3 (Kb = 1.8 x 10–5 moldm–3)
[OH–] = [(1.8 x 10–5)(0.100)]1/2 = 1.34 x 10–3 mol dm–3 pOH = -log [1.34 x 10–3] = 2.9 pH = 14 - 2.9 = 11.1 |
Salt
Nature of Salt | Formed from | pH |
Neutral | strong acid + strong base | = 7 |
Acidic | strong acid + weak base | Treat as weak acid; use weak acid formula |
Basic | weak acid + strong base | Treat as weak base; use weak base formula |
In calculating the pH of a salt solution, there are two common challenges:
1. Ka/ Kb of the salt (conjugate acid/ conjugate base) is not given.
Using Ka x Kb = Kw, find Ka/ Kb of the ion from Kb/ Ka of the parent base/ acid given.
2. finding concentration of salt.
[salt] = n (limiting reagent)/ Vtotal [Vtotal = Vacid + Vbase]
Buffer
Type | Formed from | pH |
Acidic | weak acid + conjugate base | < 7 pH = pKa + log [salt]/[acid] |
Basic | weak base + conjugate acid | > 7 pOH = pKb + log [salt]/[base] |
How to calculate pH of a buffer on adding small amounts of H+ and OH–?
Note:
Titration Curve
Example: Titration of weak acid against strong base
| Titration curve | What to observe | What can be found |
1 | Initial point | pH | [H+] due to dissociation of acid |
2 | Equivalence point | volume of base pH | Given [acid], [base] can be found Given [base], [acid] can be found type of salt/ type of titration |
3 | Half equivalence point | pH | pKa of acid |
Strategy
Example: Titration of weak acid against strong base:
Titration curve | What is in solution |
Initial point | Weak acid |
Between initial point to equivalence point | Acidic buffer |
Equivalence point | Basic Salt |
Beyond equivalence point | Strong base |