DPST1031 Chap.9 Acids, Bases, Buffers and Titration Curves
Acids, Bases, Buffers and Titration Curves
Acids and bases apply the equilibrium methods of the previous chapter to the reactions students meet most often. This chapter defines Brønsted-Lowry and Lewis acids and bases, separates strength from concentration, and uses Kw, pH and pOH with the relation Ka × Kb = Kw.
A decision flow then decides which calculation a solution needs: strong acid or base, weak acid or base with an ICE table, a salt of a weak partner, or a buffer with the Henderson-Hasselbalch equation. The chapter finishes with weak acid titration curves, the half-equivalence point where pH equals pKa, the pH at equivalence and indicator choice, all of which are tested in the laboratory as well as the final.
What this chapter covers
- 01
Brønsted-Lowry and Lewis definitions
- 02
Conjugate acid and base pairs
- 03
Strong versus weak acids and bases
- 04
Kw, pH and pOH
- 05
Weak acid and weak base pH
- 06
Acidic, basic and neutral salts
- 07
Buffers and the Henderson-Hasselbalch equation
- 08
Titration curves and indicators
Worked example · free
pH of a sodium fluoride solution
- +1Na⁺ does not react; F⁻ is the anion of the weak acid HF, so it takes a proton from water and the solution is basic.
- +1Kb(F⁻) = Kw ÷ Ka = 1.0 × 10⁻¹⁴ ÷ 6.8 × 10⁻⁴ = 1.5 × 10⁻¹¹.
- +1F⁻ + H₂O ⇌ HF + OH⁻: x² ÷ 0.20 = 1.5 × 10⁻¹¹, so x = [OH⁻] = 1.7 × 10⁻⁶ M.
- +1pOH = 5.77, so pH = 14.00 − 5.77 = 8.23.
Key terms
- Brønsted-Lowry acid
- A species that donates a proton to another species.
- Conjugate base
- The species left after an acid has donated its proton.
- Lewis acid
- A species that accepts an electron pair to form a new bond.
- Acid dissociation constant
- The equilibrium constant Ka for the ionisation of a weak acid in water.
- Ion product of water
- The constant Kw = [H₃O⁺][OH⁻], equal to 1.0 × 10⁻¹⁴ at 25 °C.
- Equivalence point
- The point in a titration where the added base exactly neutralises the acid present.
- Buffer capacity
- The amount of strong acid or base a buffer can absorb before its pH changes markedly.
Acids, Bases, Buffers and Titration Curves FAQ
What is the difference between a strong acid and a concentrated acid?
Strength describes how completely an acid ionises in water; concentration describes how much acid is dissolved per litre. A strong acid ionises completely at any concentration, while a weak acid ionises only partly even when concentrated, so the two ideas must be judged separately.
Why is pH equal to pKa at the half-equivalence point?
Halfway to equivalence, half the weak acid has been converted to its conjugate base, so their concentrations are equal. In the Henderson-Hasselbalch equation the log of the ratio is then zero, leaving pH = pKa. Reading this point off a titration curve is a standard way to measure pKa.
Why is the equivalence point above 7 for a weak acid?
At equivalence all of the weak acid has become its conjugate base, which is itself a weak base. That base reacts slightly with water to produce hydroxide ions, so the solution is basic. For a strong acid with a strong base the equivalence point is at pH 7.
How do I choose the right indicator?
Pick an indicator whose colour change falls within the steep part of the titration curve around the equivalence point. For a weak acid titrated with a strong base that region is basic, so phenolphthalein suits; for a weak base with a strong acid it is acidic, so methyl red or methyl orange is better.
How do I make a buffer from a weak acid and a strong base?
Add less strong base than the weak acid needs for neutralisation. The base converts part of the acid into its conjugate base, leaving a mixture of both. For example, adding 0.050 mol of NaOH to 0.100 mol of acetic acid leaves equal amounts of acid and acetate, a buffer with pH equal to pKa.
Exam move
Begin by sorting twenty species into strong acid, weak acid, strong base, weak base, acidic salt, basic salt or neutral salt, because classification decides every later step. Then practise one calculation of each type, writing the method name above your working. For mixtures, always do the neutralisation stoichiometry in moles first, then decide whether a buffer, an excess of strong species or a pure conjugate remains.
Finally, sketch a weak acid titration curve from memory and label the starting pH, the buffer region, the half-equivalence point and the equivalence point with the method that gives each.
Working through Acids, Bases, Buffers and Titration Curves in DPST1031? Sia is AskSia’s AI Chemistry tutor — ask any DPST1031 Acids, Bases, Buffers and Titration Curves question and get a clear, step-by-step explanation grounded in how DPST1031 is taught and assessed. Read this chapter free, then take your hardest questions to Sia.
