CCEA GCE Chemistry (1110) · Unit AS 3: Basic Practical Chemistry
Mini-Lesson
AS 3: Basic Practical Chemistry
Unit AS 3 is where the theory has to survive contact with a burette. It assesses your practical skills: volumetric analysis, gravimetric analysis, calorimetry and qualitative testing.
The examinable skill is not just doing the experiment — it is justifying every step, quantifying the uncertainty and judging whether the conclusion is actually supported by the numbers.
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
Making a standard solution
A standard solution has an accurately known concentration. The routine:
Weigh the solid accurately on a balance (weighing by difference removes the error from solid left in the weighing boat).
Dissolve it fully in a small volume of distilled water in a beaker.
Transfer to a volumetric flask, and rinse the beaker and stirring rod into the flask — otherwise some solute is left behind and the concentration is too low.
Make up to the graduation mark, with the bottom of the meniscus on the line, at eye level. Stopper and invert to mix thoroughly.
c = n ÷ Vand n = m ÷ M — so m = c × V × M
Calculate
Your turn — calculation 1
1Calculate the mass of anhydrous sodium carbonate, Na₂CO₃ (M = 106.0), needed to make 250 cm³ of a 0.100 mol dm⁻³ standard solution. Give your answer to 3 significant figures.
g
Hint: n = 0.100 × 0.250 = 0.0250 mol. mass = n × M.
Method
n = c × V = 0.100 × 0.250 = 0.0250 mol. mass = 0.0250 × 106.0 = 2.65 g.
Volumetric analysis
Titration technique — and why each step matters
Rinse the burette with the solution it will hold. Water left inside would dilute it, so a larger volume would be needed and the titre would be too high.
Rinse the pipette with the solution it will deliver, for the same reason.
The conical flask may be rinsed with distilled water — extra water changes the volume but not the moles of substance being titrated, so it has no effect on the titre.
Run a rough titration, then repeat carefully, adding dropwise near the end-point.
Take the mean of concordant titres only — within 0.10 cm³ of each other. Never include the rough titration or an anomaly.
Choosing an indicator: the colour change must fall entirely within the vertical section of the titration curve. Strong acid–strong base: either works. Strong acid–weak base (e.g. HCl with Na₂CO₃): methyl orange. Weak acid–strong base (e.g. ethanoic acid with NaOH): phenolphthalein.
Calculate
Your turn — calculation 2
225.0 cm³ of 0.100 mol dm⁻³ Na₂CO₃ is titrated against HCl. The mean titre is 22.50 cm³. Na₂CO₃ + 2HCl → 2NaCl + H₂O + CO₂. Calculate the concentration of the HCl in mol dm⁻³, to 3 significant figures.
mol dm⁻³
Hint: n(Na₂CO₃) = 0.0250 × 0.100 = 2.50 × 10⁻³ mol. The ratio is 1 : 2, so n(HCl) = 5.00 × 10⁻³. Then c = n ÷ 0.0225.
A burette reads to ±0.05 cm³. A titre needs two readings, so its uncertainty is ±0.10 cm³.
A thermometer reading to ±0.5 °C gives an uncertainty of ±1.0 °C in ΔT.
When values are multiplied or divided, add the percentage uncertainties.
Precise is not the same as accurate. Three titres agreeing to 0.05 cm³ are highly precise. If the burette is miscalibrated, they can all be equally wrong. Repeating reduces random error; only fixing the method removes a systematic one.
Calculate
Your turn — calculation 4
4A burette reading has an uncertainty of ±0.05 cm³. Calculate the percentage uncertainty in a titre of 22.50 cm³, to 2 decimal places.
Uncertainty in the titre = 2 × 0.05 = ±0.10 cm³. (0.10 ÷ 22.50) × 100 = 0.44%. A larger titre would reduce this — one reason a 25 cm³ aliquot is chosen to give a titre near 25 cm³.
Gravimetric analysis
Water of crystallisation and heating to constant mass
Heating a hydrated salt drives off the water of crystallisation. Comparing the masses before and after gives the formula:
CuSO₄·5H₂O(s) → CuSO₄(s) + 5H₂O(g)blue crystals → white anhydrous powder
Weigh the crucible, then the crucible + hydrated salt.
Heat gently, cool in a desiccator, and re-weigh.
Repeat until two consecutive masses agree — heating to constant mass proves all the water has gone.
Two systematic errors to name: heating too strongly can decompose the anhydrous salt (giving a mass loss that is too large); cooling in open air lets the hygroscopic anhydrous solid reabsorb water (giving a mass loss that is too small). A desiccator solves the second.
Calculate
Your turn — calculation 5
52.50 g of hydrated copper(II) sulfate, CuSO₄·5H₂O (M = 249.6), is heated to constant mass. Calculate the mass of anhydrous CuSO₄ (M = 159.6) that should remain, to 3 significant figures.
g
Hint: n(CuSO₄·5H₂O) = 2.50 ÷ 249.6. The ratio is 1 : 1, so n(CuSO₄) is the same. mass = n × 159.6.
Method
n = 2.50 ÷ 249.6 = 0.01002 mol. mass of CuSO₄ = 0.01002 × 159.6 = 1.60 g. The mass lost as water is 2.50 − 1.60 = 0.90 g, which is 0.0501 mol — five times the moles of salt, confirming the ·5H₂O.
NH₄⁺ — warm with NaOH(aq): ammonia is released, turning damp red litmus blue.
Anions
Carbonate — add dilute acid: effervescence, and the CO₂ turns limewater milky.
Sulfate — acidify with dilute HCl, then add BaCl₂(aq): a white precipitate of BaSO₄.
Halides — acidify with dilute HNO₃, then add AgNO₃(aq): Cl⁻ white, Br⁻ cream, I⁻ yellow. Confirm with ammonia: AgCl dissolves in dilute NH₃, AgBr only in concentrated NH₃, AgI in neither.
Gases — O₂ relights a glowing splint · H₂ gives a squeaky pop · CO₂ turns limewater milky · NH₃ turns damp red litmus blue · Cl₂ bleaches damp litmus.
Order matters. Test for carbonate first, then sulfate, then halide. Carbonate would give a precipitate with Ba²⁺ (BaCO₃ is white) and with Ag⁺, so it must be removed by acidifying first — and that acid must be the right one: use dilute HNO₃ before AgNO₃, never HCl, or you will precipitate AgCl from the acid itself.
Sort it
What is being tested for?
Tap a test result, then tap what kind of species it identifies.
🔵 A cation
🟡 An anion
💨 A gas
Quick check
Think it through
?Before testing for a halide with silver nitrate, the solution is acidified. Which acid must be used, and why?
Match it
Match the apparatus to its purpose
Tap a piece of apparatus on the left, then its purpose on the right.
Apparatus
Purpose
Quick check
Think it through
?A student rinses the conical flask with distilled water before adding the aliquot from the pipette. What effect does this have on the titre?
Quick check
Think it through
?In a calorimetry experiment the measured enthalpy change is consistently less exothermic than the data book value. Which improvement addresses the cause?
Quick check
Think it through
?Why is a hydrated salt heated to constant mass rather than for a fixed time?
Recap
The big ideas to know
Standard solution: weigh by difference, dissolve, rinse the beaker into the volumetric flask, make up to the mark at eye level
Titration: rinse the burette and pipette with their own solution; the conical flask may be wet; use concordant titres (within 0.10 cm³) only
Indicators: methyl orange for strong acid–weak base; phenolphthalein for weak acid–strong base
Uncertainty: % uncertainty = (uncertainty ÷ value) × 100; two readings double it; add percentages when multiplying
Gravimetric: heat to constant mass; cool in a desiccator
Qualitative: carbonate → sulfate → halide, in that order; acidify with HNO₃ before AgNO₃
Error: repeating reduces random error only — a systematic error needs the method fixed
That is Unit AS 3 — the practical skills, and the reasoning behind every step. Press Finish to see your score.
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