Key Definitions
- Standard solution: a solution of precisely known concentration (e.g. “1M NaOH solution”).
- Molar solution: a solution containing exactly one mole of solute per litre (1 dm³).
- Acid (Brønsted-Lowry): a proton (H⁺) donor — e.g. HCl, H₂SO₄, H₃PO₄, CH₃COOH, HNO₃, HNO₂.
- Strong acid: ionises completely in water — HCl(aq) → H⁺(aq) + Cl⁻(aq).
- Weak acid: ionises only partially; the reaction is reversible — CH₃COOH(aq) ⇌ CH₃COO⁻(aq) + H⁺(aq).
- Basicity of an acid: the number of replaceable hydrogen ions per molecule.
- Base (Brønsted-Lowry): a proton acceptor — usually a metal oxide or hydroxide, e.g. Na₂O, KOH, CaCO₃, NaOH, aqueous ammonia (NH₃/NH₄OH).
- Alkali: a base that’s soluble in water.
- Strong alkali: ionises completely (irreversibly) — NaOH(aq) → Na⁺(aq) + OH⁻(aq).
- Weak alkali: ionises only partially — NH₄OH(aq) ⇌ NH₄⁺(aq) + OH⁻(aq).
| Acid | Ions produced | Basicity |
|---|---|---|
| CH₃COOH | CH₃COO⁻ and H⁺ | 1 (monobasic) |
| HCl | H⁺ and Cl⁻ | 1 (monobasic) |
| H₂SO₄ | 2H⁺ and SO₄²⁻ | 2 (dibasic) |
| HNO₃ | H⁺ and NO₃⁻ | 1 (monobasic) |
| H₃PO₄ | 3H⁺ and PO₄³⁻ | 3 (tribasic) |
Preparing a Standard Solution
Worked example: preparing 250cm³ of a 0.1M solution of anhydrous sodium carbonate (molar mass 106 g/mol).
Required mass: m = M × MM × V, with V converted to dm³ (250cm³ = 0.025 dm³): m = 0.1 mol/dm³ × 106 g/mol × 0.025 dm³ = 2.65 g.
Procedure: weigh 2.65g of anhydrous sodium carbonate accurately in a dry weighing bottle, transfer to a clean beaker, dissolve completely in about 100cm³ of distilled water, then transfer quantitatively into a 250cm³ volumetric flask through a funnel — washing the weighing bottle, stirrer and funnel into the flask too, so no solute is lost. Make up to the calibration mark with distilled water, stopper, and invert repeatedly to mix into a homogeneous solution.
Carrying Out a Titration
Titration is finding the concentration of one solution by reacting it, drop by measured drop (usually from a burette), against a standard solution of known concentration — typically tracked with an indicator’s colour change.
Worked example: standardising hydrochloric acid against 0.1M sodium carbonate solution, using methyl orange indicator. A pipette, burette and three conical flasks are rinsed with distilled water and then with the relevant solution (sodium carbonate for the pipette, acid for the burette). The burette is clamped, filled with acid (no trapped air bubbles), and its starting level recorded. 25cm³ of the 0.1M Na₂CO₃ solution is pipetted into each conical flask, with two or three drops of indicator added. Acid is run in from the burette — quickly at first for a rough (“approximate”) endpoint, then drop by drop near the endpoint on repeat (“accurate”) runs — until one drop produces a permanent colour change. The volume used (the titre) is recorded each time.
Results are tabulated (first and second burette readings, and the titre for the approximate run and two accurate runs), and the average titre is taken as the mean of the two accurate runs.
Worked Calculations
For a reaction aA + bB → products, where a and b are the balancing coefficients of the acid and base, the general titration relationship is: (MaVa)/a = (MbVb)/b.
1. What volume of 2M HCl neutralises 25cm³ of 0.4M NaOH? HCl(aq) + NaOH(aq) → NaCl(aq) + H₂O(l) is a 1:1 reaction, so MaVa = MbVb. Va = MbVb/Ma = (0.4 × 25)/2 = 5 cm³.
2. What volume of 0.05M Na₂CO₃ neutralises 25.0cm³ of 0.1M HCl? 2HCl(aq) + Na₂CO₃(aq) → 2NaCl(aq) + H₂O(l) + CO₂(g) needs 2 mol acid per 1 mol carbonate, so MaVa = 2MbVb. Vb = MaVa/(2Mb) = (0.1 × 25)/(2 × 0.05) = 25 cm³.
3. 25cm³ of 1.0M KOH neutralises 60cm³ of sulphuric acid. Find (a) the acid’s molarity and (b) its concentration in g/cm³. H₂SO₄(aq) + 2KOH(aq) → K₂SO₄(aq) + 2H₂O(l) needs 2 mol base per 1 mol acid, so 2MaVa = MbVb.
(a) Ma = MbVb/(2Va) = (1.0 × 25)/(2 × 60) ≈ 0.208 M.
(b) Molar mass of H₂SO₄ = (2×1) + (1×32) + (4×16) = 98 g/mol. Concentration in g/dm³ = molarity × molar mass = 0.208 × 98 ≈ 20.384 g/dm³. Converting to g/cm³ (÷1000, since 1 dm³ = 1000 cm³): ≈ 0.0204 g/cm³.
