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You have now worked through the whole of Topic C5 of OCR Gateway Science A — concentration of solutions, titrations and titration calculations, percentage yield, atom economy, gas volumes, rates of reaction and collision theory, measuring and calculating rates, and reversible reactions and equilibrium. This final lesson pulls it all together. It shows how the C5 ideas connect, gathers the calculation toolkit in one place, revisits the required practicals, drills the misconceptions that catch students out, and finishes with a synoptic model answer. Treat it as a revision and exam-technique session rather than new content.
By the end of this lesson you should be able to recall and use every C5 calculation, describe the C5 required practicals, reason confidently about rates and equilibrium, avoid the common C5 errors, and structure a top-band answer.
C5 is the most calculation-heavy topic in GCSE chemistry, so the single most valuable habit you can take into the exam is to set out every calculation clearly and check your units. This lesson gathers all of those calculations in one place so you can practise them as a set.
It helps to see C5 as one connected story. Chemists monitor reactions by measuring amounts — concentrations, yields, gas volumes — and they control reactions by changing the rate and shifting the equilibrium.
flowchart TD
A["Monitoring and controlling reactions"] --> B["Measuring amounts"]
A --> C["Controlling rate"]
A --> D["Controlling equilibrium"]
B --> E["Concentration<br/>(g/dm3, mol/dm3)"]
B --> F["Titrations<br/>+ calculations"]
B --> G["Yield, atom economy<br/>gas volumes"]
C --> H["Collision theory<br/>temp, conc, surface area, catalyst"]
D --> I["Reversible reactions<br/>dynamic equilibrium"]
I -.->|"Le Chatelier"| J["Haber process"]
H -.->|"catalyst lowers Ea"| I
Notice the links: a catalyst appears in both rate (it lowers the activation energy) and equilibrium (it speeds both directions equally, reaching equilibrium faster). Concentration runs through titrations, rate and Le Chatelier. Seeing these connections is exactly the synoptic thinking that lifts an answer.
Several calculation types recur in C5. Here they are in one reference table, then each with a fresh worked example.
| Quantity | Formula | Unit |
|---|---|---|
| Mass concentration | conc=volume (dm3)mass | g/dm³ |
| Molar concentration (H) | conc=volume (dm3)moles | mol/dm³ |
| Convert (H) | g/dm3=mol/dm3×Mr | — |
| Percentage yield | theoreticalactual×100 | % |
| Atom economy (H) | total Mr reactantsMr desired product×100 | % |
| Gas volume (H) | volume=moles×24 | dm³ (rtp) |
| Mean rate | timequantity changed | cm³/s, g/s, mol/s |
(H = Higher tier.)
Worked example: Find the concentration in g/dm³ of 5 g of solute in 250 cm3. Convert: 250 cm3=0.25 dm3; conc=0.255=20 g/dm3.
Worked example: 25.0 cm3 of NaOH is neutralised by 24.0 cm3 of 0.100 mol/dm3 HCl (1:1). Find the NaOH concentration. Moles HCl =0.100×0.0240=0.00240; ratio 1:1, so moles NaOH =0.00240; conc=0.02500.00240=0.0960 mol/dm3.
Worked example: Theoretical yield 16 g, actual yield 12 g. % yield=1612×100=75%.
Worked example: Atom economy for ammonia, N2+3H2→2NH3. Desired 2NH3=34; reactants 28+6=34; 3434×100=100% (single product).
Worked example: Volume at rtp of 0.2 mol of CO2. volume=0.2×24=4.8 dm3.
Worked example: 60 cm3 of gas in 40 s. mean rate=4060=1.5 cm3/s.
Exam Tip: Show three lines in every calculation — equation, substitution, answer with unit — and convert cm³ to dm³ (÷1000) before any concentration or moles step. For titrations, read the mole ratio from the balanced equation.
| Practical | What you do | Key technique | Top marks come from |
|---|---|---|---|
| Titration | Find the volume of acid that neutralises a measured volume of alkali | Pipette + burette, single-change indicator | Swirling; drop by drop at the end; concordant titres; mean (ignore rough) |
| Rate of reaction | Follow a quantity over time (gas volume / mass loss / disappearing cross) | Gas syringe, balance, or "disappearing cross" | Correct method for the reaction; reading the gradient; controlling variables |
Exam Tip: For each practical, examiners reward the independent variable (what you change), the dependent variable (what you measure) and the control variables (what you keep the same). For a rate investigation, control the concentration, volume and temperature of the other reactant.
OCR uses specific command words that tell you exactly what kind of answer to give.
| Command word | What it asks for |
|---|---|
| State / Name / Give | A short fact, no explanation (e.g. name the molar gas volume) |
| Describe | Say what happens (e.g. describe how to do a titration) |
| Explain | Give reasons why — use "because", "so that" (e.g. why a powder reacts faster) |
| Calculate | Work out a number — show working and give a unit (e.g. a concentration) |
| Predict | Use the rules to say what will happen (e.g. the effect of pressure on an equilibrium) |
| Compare | Give the similarities and differences (e.g. two routes by atom economy) |
Exam Tip: The difference between describe and explain decides many marks. "The rate increases" describes; "...because collisions are more frequent" explains. If the command word is explain, you must give the reason.
A reliable routine for any data question is: (1) read the headings and units of the table or axes; (2) describe the pattern in words (rises, falls, levels off); (3) quote figures to support what you say; and (4) if asked to explain, give the chemistry behind the pattern (for example, a rate–time curve levels off because a reactant has run out, and is steepest at the start because the concentration is highest). Quoting figures turns a vague description into an evidenced one, which examiners reward.
Use this as a final recall list. Cover the right-hand column and test yourself.
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