CCEA GCE Chemistry (1110) · Unit A2 3: Further Practical Chemistry
Mini-Lesson
A2 3: Further Practical Chemistry
Unit A2 3 takes practical work to A2 standard: redox and back titrations, full organic preparation and purification, and quantitative techniques such as colorimetry.
The examinable skill is judgement — choosing the right technique, quantifying the uncertainty, and deciding what the data will and will not support.
Work through each screen, answer the questions as you go (several are full A-level calculations) and collect ⭐ stars. Press Start when you're ready.
Volumetric analysis
The back titration
A back titration is used when the substance is insoluble, impure or reacts too slowly to titrate directly — an indigestion tablet, an impure limestone chip, an antacid.
Add a known excess of acid and let it react completely with the sample.
Titrate the unreacted (excess) acid with a standard alkali.
Subtract: moles reacted with the sample = moles added − moles left over.
The excess must be genuine. If you add too little acid the sample will not dissolve fully and the whole calculation is invalid. If you add far too much, the titre becomes a small difference between two large numbers — and the percentage uncertainty rockets. A moderate, verified excess is the goal.
Calculate
Your turn — calculation 1
10.500 g of impure CaCO₃ is dissolved in 50.0 cm³ of 0.500 mol dm⁻³ HCl (an excess). CaCO₃ + 2HCl → CaCl₂ + H₂O + CO₂. The excess HCl needs 32.0 cm³ of 0.500 mol dm⁻³ NaOH. Calculate the percentage purity of the CaCO₃, to the nearest whole number. (M(CaCO₃) = 100.1)
%
Hint: n(HCl) added = 0.0500 × 0.500. n(NaOH) = 0.0320 × 0.500 = n(HCl) left over. Subtract, then ÷ 2 for n(CaCO₃), then × 100.1 and compare with 0.500 g.
?Why is a back titration used to analyse an indigestion tablet rather than a direct titration?
Organic preparation
Reflux, distillation and purification
A typical organic preparation runs through four stages:
Reaction — heat under reflux (a vertical condenser). Vapour condenses and returns to the flask, so volatile reactants are not lost and the mixture can be heated for a long time. Add anti-bumping granules to prevent violent boiling.
Separation — distillation collects the product at its boiling point; a separating funnel removes an immiscible aqueous layer.
Washing and drying — wash with sodium carbonate solution to remove acid (vent the funnel — CO₂ is produced!), then dry with an anhydrous salt such as anhydrous sodium sulfate or magnesium sulfate.
Purification — recrystallisation for a solid: dissolve in the minimum volume of hot solvent, filter hot to remove insoluble impurities, cool slowly to crystallise the product, then filter under suction and wash with cold solvent.
Why recrystallisation works: the desired product is much less soluble in the cold solvent, so it crystallises out. The soluble impurities are present in small amounts and stay dissolved in the cold solvent. Washing with cold solvent removes the last of them without redissolving much of the product.
Checking purity: a pure solid melts sharply, at the literature value. Impurities lower the melting point and broaden the range.
Calculate
Your turn — calculation 2
2Aspirin is prepared from 2.00 g of 2-hydroxybenzoic acid (M = 138.0). The theoretical product is aspirin (M = 180.0), in a 1 : 1 ratio. 1.80 g of purified aspirin is obtained. Calculate the percentage yield, to the nearest whole number.
%
Hint: n(acid) = 2.00 ÷ 138.0 = 0.01449 mol → theoretical mass of aspirin = 0.01449 × 180.0. Then (1.80 ÷ theoretical) × 100.
Method
n = 2.00 ÷ 138.0 = 0.01449 mol. Theoretical mass = 0.01449 × 180.0 = 2.61 g. % yield = (1.80 ÷ 2.61) × 100 = 69%. Losses come from the reversible reaction, transfers between vessels, and product left dissolved in the recrystallisation solvent.
Quick check
Think it through
?A student recrystallises a product and measures its melting point as 132–137 °C. The literature value is 138–139 °C. What does this tell you?
Quick check
Think it through
?Why is a solid dissolved in the MINIMUM volume of HOT solvent during recrystallisation?
Sort it
Which experiment is this from?
Tap an item of apparatus or a step, then tap the type of experiment it belongs to.
💧 Titration
⚗️ Organic preparation
⏱️ Rate experiment
Uncertainty
Percentage uncertainty and evaluation
% uncertainty = (uncertainty ÷ measured value) × 100when values are multiplied or divided, add their percentage uncertainties
Burette: ±0.05 cm³ per reading → ±0.10 cm³ in a titre (two readings).
Thermometer: ±0.1 °C per reading → ±0.2 °C in ΔT.
Balance: ±0.005 g per reading → ±0.01 g in a difference weighing.
How to actually improve an experiment: to cut the percentage uncertainty you either use apparatus with a smaller uncertainty, or make the measured value larger (a bigger titre, a bigger temperature rise, a bigger mass). Vague answers like be more careful or repeat it more times score nothing against a systematic error.
Calculate
Your turn — calculation 3
3A thermometer has an uncertainty of ±0.1 °C per reading. In a calorimetry experiment ΔT is measured as 8.0 °C. Calculate the percentage uncertainty in ΔT.
%
Hint: ΔT needs two readings → ±0.2 °C. (0.2 ÷ 8.0) × 100.
Method
Uncertainty in ΔT = 2 × 0.1 = ±0.2 °C. (0.2 ÷ 8.0) × 100 = 2.5%. Using a bigger temperature rise (more concentrated reagents) would reduce this.
Calculate
Your turn — calculation 4
425.0 cm³ of diluted vinegar is titrated against 0.100 mol dm⁻³ NaOH; the mean titre is 21.0 cm³. CH₃COOH + NaOH → CH₃COONa + H₂O. Calculate the concentration of ethanoic acid in the diluted vinegar, in mol dm⁻³, to 3 significant figures.
mol dm⁻³
Hint: n(NaOH) = 0.0210 × 0.100 = 2.10 × 10⁻³ mol. The ratio is 1 : 1. c = n ÷ 0.0250.
Colorimetry measures the absorbance of a coloured solution. Absorbance is proportional to concentration, so a calibration curve built from standards of known concentration lets you read off an unknown.
Choose the filter of the complementary colour to the solution (a blue Cu²⁺ solution absorbs red light, so use a red filter) — this maximises absorbance and therefore sensitivity.
Zero the instrument with a blank (pure solvent) so that only the solute is measured.
Rate experiments follow a property that changes with time: gas volume (gas syringe), mass loss (balance), colour (colorimeter), or the time to reach a fixed point (the classic thiosulfate disappearing cross).
Initial rate matters. The rate falls as reactants are used up, so a mean rate over the whole reaction understates the start. The initial rate is found from the gradient of the tangent at t = 0, and it is the initial rate that lets you find the order in each reactant.
Calculate
Your turn — calculation 5
5In a colorimetry calibration, absorbance is directly proportional to concentration. A 0.0500 mol dm⁻³ standard gives an absorbance of 0.400. An unknown sample gives an absorbance of 0.240. Calculate its concentration in mol dm⁻³.
mol dm⁻³
Hint: Absorbance is proportional to concentration, so c = 0.0500 × (0.240 ÷ 0.400).
A risk assessment is not a list of hazards — it must pair each hazard with the risk it creates and the control measure that reduces it.
Concentrated sulfuric acid — corrosive. Wear eye protection and gloves; add acid to water, never water to acid.
Organic solvents (ethanol, ether) — highly flammable. Heat with an electric heating mantle or water bath, never a naked flame.
KCN / HCN — toxic. Use a fume cupboard, and never acidify a cyanide.
Bromine, chlorine — toxic and corrosive vapours. Fume cupboard.
What makes a method valid: only one independent variable is changed; every other factor that could affect the result is controlled; the measurements are made with apparatus of adequate resolution; and enough repeats are taken to identify anomalies. A method that produces beautifully precise data while a control variable drifts is still invalid.
Match it
Match the step to the reason
Tap a practical step on the left, then the reason for it on the right.
Step
Reason
Quick check
Think it through
?Why must the acid used in a manganate(VII) redox titration be dilute sulfuric acid, not hydrochloric or nitric acid?
Quick check
Think it through
?A calibration curve for a colorimeter is plotted using five standard solutions. Why is a blank (pure solvent) used to zero the instrument first?
Recap
The big ideas to know
Back titration: add a known excess, titrate what is left, subtract — used for insoluble, impure or slow-reacting samples
Organic prep: reflux (no loss of volatiles, anti-bumping granules) → distil → wash and dry → recrystallise
Recrystallisation: minimum volume of hot solvent; filter hot; cool slowly; wash with cold solvent
Purity: a pure solid melts sharply at the literature value; impurities lower and broaden it
Uncertainty: % uncertainty = (uncertainty ÷ value) × 100; add percentages when multiplying or dividing
Redox titration: acidify with dilute H₂SO₄ only; MnO₄⁻ is self-indicating
Colorimetry: zero on a blank, use the complementary filter, read from a calibration curve
Rates: the initial rate is the gradient of the tangent at t = 0
That is Unit A2 3 — and with it, the whole practical strand of CCEA Chemistry. Press Finish to see your score.
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