Cameroon GCE O-Level Chemistry common mistakes
Cameroon GCE O-Level Chemistry · Understand the traps and how to avoid them.
Writing unbalanced symbol equations, or copying the word equation straight across without inserting coefficients (e.g. writing H2 + O2 → H2O and then using it in a mole calculation).
Candidates learn the word equation first and treat the formula equation as a translation exercise; they do not habitually check atom counts on both sides, and they forget diatomic elements (H2, O2, N2, Cl2, F2, Br2, I2) exist as molecules.
What to do instead
Adopt a fixed routine: write correct formulae, then count each element left and right in a small table, adjust coefficients only (never subscripts), and finish by re-counting. Practise 10 equations per session covering combustion, neutralisation, displacement and precipitation until counting is automatic.
Treating the mole as a mass. For example, stating that 1 mole of CO2 has a mass of 44 g and therefore contains 44 molecules, or adding masses directly to find moles in a limiting-reagent problem without dividing by molar mass.
The mole is introduced alongside relative molecular mass, so the number 44 becomes fused with 'one mole' in the candidate's mind; the distinction between amount (mol), mass (g) and number of particles is never made explicit.
What to do instead
Keep a three-way conversion triangle visible: mass ÷ Mr = moles, moles × 6.02 × 10^23 = particles, moles × 24 dm3 = gas volume at r.t.p. For every calculation, write the quantity asked for and its unit before substituting numbers, and check the unit of the answer.
Confusing ionic and covalent descriptions: saying sodium chloride is a covalent molecule with a low melting point because 'it is made of a metal and a non-metal but shares electrons', or saying diamond conducts electricity because it has delocalised electrons.
Candidates memorise 'metal + non-metal = ionic' as a rule of thumb and then misapply it to cases like AlCl3, and they conflate the delocalised electrons of graphite with the rigid covalent lattice of diamond.
What to do instead
Build a comparison table with columns for particles present, forces broken on melting, and charge carriers (ions in melt/solution for ionic, delocalised electrons for metals and graphite, none for simple molecular and diamond). For each substance, name the particle that must move for conduction before answering.
Describing electrolysis products by the wrong electrode: stating that hydrogen is produced at the anode, or that copper is deposited at the anode during electrolysis of copper(II) sulphate with copper electrodes.
The words anode and cathode are learned as labels without a consistent meaning; candidates do not link anode to oxidation and cathode to reduction, and they do not distinguish the reactive copper anode (which dissolves) from an inert platinum anode (which liberates oxygen).
What to do instead
Fix the definitions: anode = oxidation = loss of electrons, cathode = reduction = gain of electrons. Before each electrolysis question, write the ions present, then decide which ion is discharged at each electrode and whether the electrode itself is reactive. Use OIL RIG (oxidation is loss, reduction is gain) as a check on every half-equation.
Answering 'rate of reaction' questions with the yield or the position of equilibrium, e.g. claiming a catalyst increases the yield of ammonia in the Haber process, or that raising temperature shifts equilibrium and therefore speeds up the reaction.
Rate and equilibrium are taught close together and both involve temperature, pressure and catalysts, so the candidate's vocabulary blends; the distinction between how fast equilibrium is reached and where it lies is never practised as a separate idea.
What to do instead
State the two rules explicitly and test them against each factor: a catalyst changes only rate (both forward and backward equally), never position; temperature changes both rate and position; pressure and concentration change position (for gases/solutions) and may change rate. Redo past-paper questions labelling each answer as 'rate', 'position' or 'both'.
Naming salts and writing formulae incorrectly: calling the product of hydrochloric acid and copper(II) oxide 'copper chloride' without the (II), writing CaCl instead of CaCl2, or writing sulphate as SO4 without the 2- charge.
Candidates rely on word patterns and ignore ionic charges; they have not memorised the common ions and their charges, so they cannot derive a formula from the ions present.
What to do instead
Memorise the ion table (charges on Na+, Mg2+, Al3+, Cl−, O2−, OH−, NO3−, SO4^2−, CO3^2−, NH4+) and use the 'swap the charges, cancel' method to build every formula. When naming, decide whether the metal has more than one valency and insert the Roman numeral accordingly.
In volumetric analysis, misreading the burette (reading the top of the meniscus or reading downwards from the top), and then carrying out the mole-ratio step of the titration calculation with the acid and alkali in the wrong order.
The burette scale runs from 0 at the top, unlike a measuring cylinder, and candidates read it as if it were a thermometer; in calculations they plug numbers into a memorised formula without tracking which substance is in the pipette and which in the burette.
What to do instead
Always read the bottom of the meniscus with the eye level with it, and record the initial and final readings to 0.05 cm3. In calculations, write a labelled flow: volume of pipetted solution → moles (using its concentration) → moles of the other substance via the balanced equation → concentration. Include the ratio explicitly each time.
Reporting qualitative-analysis observations as conclusions: writing 'the gas was carbon dioxide' or 'the solution contained Fe2+' instead of describing what was seen (effervescence, white precipitate soluble in excess, green precipitate).
Candidates know the answer and skip the observation, not realising that marks are awarded for the described change; they also omit the reagent and the condition (e.g. 'in excess', 'on warming').
What to do instead
Use the three-part sentence: reagent added → observation seen (colour, precipitate, gas, smell) → conclusion. Drill the cation and anion tests aloud, always stating the reagent, the initial observation and the behaviour in excess or on heating. Never write a formula as an observation.
Drawing dot-and-cross diagrams with the wrong number of outer electrons, or leaving out the charges on ions in ionic diagrams, and drawing the shared pair between the wrong atoms in a molecule such as H2O or CO2.
Candidates do not count valence electrons from the group number, and they draw diagrams as decoration rather than as a representation of electron transfer or sharing; the octet rule is quoted but not applied.
What to do instead
Start every diagram by writing the group number of each atom and hence the number of outer electrons. For ionic compounds, draw square brackets around each ion with its charge shown. For covalent compounds, place the shared pair clearly in the overlap region and count that each atom reaches a full outer shell. Practise H2O, NH3, CH4, CO2, NaCl, MgO and MgCl2.
Confusing the three states and kinetic-theory explanations: saying particles in a liquid are far apart and move randomly like a gas, or explaining expansion on heating as 'particles getting bigger' rather than moving further apart.
The particle model is learned as a diagram of dots, and candidates describe the picture rather than the behaviour; the phrases 'particles expand' and 'particles melt' are absorbed from everyday speech.
What to do instead
Learn one sentence per state covering arrangement, spacing and motion, and use the same three headings in every answer. When explaining a change of state or gas pressure, always write about the speed and spacing of particles and the forces between them, never about the particles themselves changing size. Apply the same wording to diffusion and Brownian motion questions.