OCR A-level Chemistry A (H432) · Module 1: Development of practical skills in chemistry
Mini-Lesson
Development of Practical Skills
Module 1 is the practical backbone of OCR Chemistry A. It is not examined as a separate paper — it is threaded through every written paper, so questions on planning, implementing, analysis and evaluation can appear anywhere.
You will meet variables, apparatus choice, systematic vs random error, resolution, percentage uncertainty, concordant titres, significant figures and how to judge whether a conclusion is actually supported by the data.
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.
Planning
Variables, hypotheses and a valid method
A valid experiment changes one thing and holds the rest constant.
Independent variable — the one you deliberately change (e.g. concentration of HCl).
Dependent variable — the one you measure (e.g. volume of H₂ produced in 60 s).
Control variables — everything held constant (temperature, mass and surface area of Mg, total volume).
Apparatus resolution matters. A 50 cm³ measuring cylinder reads to ±0.5 cm³; a burette reads to ±0.05 cm³. Choosing apparatus with a higher resolution lowers the percentage uncertainty in your result — this is the single most common 'suggest an improvement' mark.
A hypothesis must be testable: the initial rate is proportional to [HCl] is testable; stronger acid works better is not.
Implementing · titration
Titration technique — every step has a reason
Rinse the burette with the solution it will contain — residual water would dilute it and give a falsely high titre.
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 number of moles being titrated, so the titre is unaffected.
Do a rough titration first, then repeat until you have concordant titres (within 0.10 cm³).
Mean the concordant titres only — never include the rough or an anomaly.
n = c × Vwith c in mol dm⁻³ and V in dm³ (divide a cm³ volume by 1000)
Quick check
Think it through
?A student rinses the burette with distilled water and fills it with 0.100 mol dm⁻³ HCl without rinsing it with the acid. What is the effect on the titre?
Analysis · uncertainty
Uncertainty and percentage uncertainty
Every measurement carries an uncertainty set by the resolution of the instrument.
A burette has an uncertainty of ±0.05 cm³ per reading. A titre needs two readings (start and end), so the uncertainty in the titre is ±0.10 cm³.
A two-decimal-place balance reading to ±0.005 g used for a difference weighing also doubles to ±0.01 g.
A thermometer with ±0.5 °C used to find ΔT (two readings) gives ±1.0 °C.
The fix examiners want: to reduce percentage uncertainty, either use apparatus with a smaller uncertainty, or make the measured value bigger (a larger titre, a bigger temperature rise, a larger mass).
Calculate
Your turn — calculation 1
1A burette reading has an uncertainty of ±0.05 cm³. A titre of 24.00 cm³ is recorded. Calculate the percentage uncertainty in the titre, to 2 significant figures.
Uncertainty in the titre = 2 × 0.05 = ±0.10 cm³. % uncertainty = (0.10 ÷ 24.00) × 100 = 0.42%.
Calculate
Your turn — calculation 2
2A balance has an uncertainty of ±0.005 g on a single reading. A student weighs out 1.250 g of solid in one weighing. Calculate the percentage uncertainty in the mass.
%
Hint: (0.005 ÷ 1.250) × 100.
Method
(0.005 ÷ 1.250) × 100 = 0.40%. Weighing a larger mass would reduce this.
Calculate
Your turn — calculation 3
3Titres: 24.10, 24.05, 24.60 and 24.15 cm³. Using only the concordant titres (within 0.10 cm³ of each other), calculate the mean titre in cm³ to 2 decimal places.
cm³
Hint: 24.60 is anomalous — discard it. Mean of 24.10, 24.05 and 24.15.
Method
(24.10 + 24.05 + 24.15) ÷ 3 = 72.30 ÷ 3 = 24.10 cm³. The 24.60 result is an anomaly and is excluded.
Quick check
Think it through
?In the titration above, the 24.60 cm³ result is an anomaly. Which statement best describes the correct response?
Calculate
Your turn — calculation 4
4In a calorimetry experiment the thermometer has an uncertainty of ±0.5 °C. The temperature rise measured is 20.0 °C. Calculate the percentage uncertainty in ΔT.
%
Hint: ΔT needs two readings → ±1.0 °C. (1.0 ÷ 20.0) × 100.
Method
Uncertainty in ΔT = 2 × 0.5 = ±1.0 °C. (1.0 ÷ 20.0) × 100 = 5.0%.
Analysis · combining uncertainties
Adding up the uncertainty in a whole experiment
When quantities are multiplied or divided in a calculation, you add the percentage uncertainties of every measurement used.
total % uncertainty = Σ (% uncertainty of each measurement)
A standard-solution titration uses three pieces of apparatus:
Balance (weighing the solid) — 0.40%
Volumetric flask (making it up to the mark) — e.g. ±0.30 cm³ in 250 cm³
Burette (the titre) — 0.42%
Significant figures: quote your final answer to the same number of significant figures as the least precise measurement. A titre of 24.00 cm³ (4 s.f.) and a concentration of 0.100 mol dm⁻³ (3 s.f.) → answer to 3 s.f.
Calculate
Your turn — calculation 5
5A 250 cm³ volumetric flask has an uncertainty of ±0.30 cm³. Combine its percentage uncertainty with the balance (0.40%) and the burette (0.42%) from the questions above to get the total percentage uncertainty, to 2 decimal places.
Systematic error — shifts every reading by the same amount or in the same direction (an unzeroed balance, heat loss). Repeating does not remove it; only recalibration or a better method does.
Random error — scatters readings either side of the true value. Repeating and averaging reduces its effect.
Gross error (mistake) — a misread or mis-recorded value; the result is simply wrong.
Accuracy vs precision: a set of titres of 24.85, 24.84, 24.86 cm³ is highly precise; if the burette is miscalibrated they can still all be inaccurate. Precision does not prove accuracy.
A conclusion must be supported by the data and by chemical reasoning. If the uncertainty in your result is larger than the difference you are claiming, you cannot claim it.
Sort it
Which kind of error?
Tap a card, then tap the type of error it is.
📏 Systematic
🎲 Random
💥 Gross (mistake)
Quick check
Think it through
?A calorimetry experiment always gives an enthalpy change that is less exothermic than the data-book value. Which improvement would most reduce this error?
Match it
Match the term to its meaning
Tap a term on the left, then its definition on the right.
Term
Meaning
Quick check
Think it through
?A student measures 25.0 cm³ of acid. Which piece of apparatus gives the smallest percentage uncertainty for this volume?
Analysis · recording data
Significant figures, tables and graphs
Recording data properly is worth marks in its own right.
Record every reading to the resolution of the instrument — a burette reading is 24.00 cm³, not 24 cm³. The trailing zeros are real information.
Every column heading needs a quantity and a unit, written as volume / cm³.
Quote a calculated answer to the same number of significant figures as the least precise measurement used in it — no more.
On a graph, plot the independent variable on the x-axis, use a scale that fills at least half the grid, and draw a line of best fit — not a dot-to-dot.
The gradient is the point. In most A-level practicals the answer is not a single reading but the gradient of a straight-line graph — for example, ln k against 1/T gives −Ea/R. Choose two points far apart on the line of best fit (never two raw data points) to calculate it.
Quick check
Think it through
?A titre of 24.00 cm³ (4 s.f.) is used with a concentration of 0.100 mol dm⁻³ (3 s.f.) and a volume of 25.0 cm³ (3 s.f.). To how many significant figures should the final concentration be quoted?
Recap
The big ideas to know
Planning: independent, dependent and control variables; a testable hypothesis; apparatus chosen for resolution
Implementing: rinse burette and pipette with their own solution; conical flask may be wet; concordant titres within 0.10 cm³
Uncertainty: % uncertainty = (uncertainty ÷ value) × 100; two readings double the uncertainty; add % uncertainties when multiplying or dividing
Error: systematic (shifts all readings, repeating will not help) · random (scatter, averaging helps) · gross (a mistake)
Evaluation: precise ≠ accurate; a conclusion must be within the uncertainty of the data
You have covered Module 1 — the practical skills OCR assess in every written paper. Press Finish to see your score.
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