MATSEC IM Chemistry common mistakes
MATSEC IM Chemistry · Understand the traps and how to avoid them.
Writing 'moles = mass × molar mass' or substituting the molar mass in place of the mass when using n = m/Mr, so a 4 g sample of CaCO3 (Mr = 100) is reported as 400 mol instead of 0.04 mol.
The formula is memorised as a string of letters without the units attached, so candidates cannot check the rearrangement by unit cancellation; under exam pressure they grab whichever numbers are visible and multiply.
What to do instead
Always write the triangle or rearrange symbolically first (n = m/Mr, m = n × Mr, Mr = m/n), then substitute with units carried through: 4 g ÷ 100 g mol⁻¹ = 0.04 mol. Do a sanity check that the answer is a plausible size for a laboratory-scale sample.
Balancing equations by adjusting the formula of a product (e.g. changing Fe + O2 → FeO into Fe + O2 → FeO2) rather than changing only the coefficients in front of intact formulae.
Candidates treat balancing as number-matching rather than as conserving atoms in fixed substances, and they have not internalised that formulae are fixed by valency while coefficients are free.
What to do instead
Adopt a fixed routine: list each element and count atoms on both sides, then change coefficients only. Never alter a subscript. Finish by re-counting every element, and check charges if it is an ionic equation.
Confusing relative atomic mass with mass number, or treating Ar values as whole numbers when calculating Mr for compounds containing Cl (35.5) or Cu (63.5), leading to wrong Mr and hence wrong mole and percentage-yield answers.
Textbook examples often use whole-number Ar, so the decimal values in the Data Booklet are ignored or rounded carelessly; the distinction between isotopes and average atomic mass is also weakly understood.
What to do instead
Use the Data Booklet Ar values exactly as printed and keep one extra decimal place through the calculation. Be able to explain that Ar is the weighted mean of isotope masses relative to 1/12 of a C-12 atom, which is why it is not a whole number.
Drawing dot-and-cross diagrams with the wrong number of outer electrons, or omitting lone pairs and formal charges, for species such as NH4+, H3O+ and CO2.
Candidates count total electrons from memory rather than from group numbers, forget to subtract or add electrons for charge, and do not check that each atom obeys the octet (or duet) rule.
What to do instead
Work systematically: sum valence electrons from group numbers, add one per negative charge and subtract one per positive charge, place bonding pairs first, then complete octets, then assign any remainder to the central atom. For CO2 show two double bonds and two lone pairs on each oxygen.
Predicting molecular shape from the number of bonded atoms only, so H2O is described as linear and NH3 as trigonal planar, without accounting for lone pairs.
The VSEPR logic (electron-pair repulsion, lone pairs repel more than bonding pairs) is learned as a label rather than as a procedure, and candidates skip counting lone pairs on the central atom.
What to do instead
Count electron domains around the central atom (bonding + lone pairs), assign the electron-domain geometry, then remove lone-pair positions to get the molecular shape and quote the bond angle: 2 domains linear 180°, 3 trigonal planar 120°, 4 tetrahedral 109.5°, with each lone pair compressing angles by roughly 2–2.5°.
Describing an ionic solid as conducting electricity 'because it contains ions' without specifying that conduction requires ions free to move, i.e. molten or in aqueous solution, not in the solid lattice.
Candidates memorise 'ionic compounds conduct' as a slogan and do not link conductivity to mobile charge carriers, so they lose the explanation mark even when the observation is right.
What to do instead
Always state the mechanism: in the solid the ions are held in a fixed lattice and cannot move; when melted or dissolved the lattice breaks down and the ions become mobile charge carriers. Contrast with metals (delocalised electrons) and covalent molecular substances (no ions, no free electrons).
Writing 'the reaction shifts to the right' when a change actually alters the value of Kc, or claiming a catalyst increases yield by shifting the position of equilibrium.
Le Chatelier's principle is applied as a phrase without distinguishing between a shift in position at constant K and a change in the equilibrium constant caused by temperature; the role of catalysts is also misremembered.
What to do instead
State explicitly that concentration and pressure changes shift position at constant K, temperature changes alter K itself (endothermic favoured on heating), and a catalyst only shortens the time to reach the same equilibrium position, with no effect on yield or K.
Calculating pH as −log[H+] without converting concentration units, or using the concentration of a weak acid directly as [H+] without an acid-dissociation calculation, giving pH values that are far too low.
The definition pH = −log[H+] is applied mechanically; candidates do not distinguish strong from weak acids or check that concentrations are in mol dm⁻³ before taking the logarithm.
What to do instead
For a strong monoprotic acid, [H+] equals the acid concentration; for a weak acid, set up Ka = [H+]²/[HA] (or use the approximation [H+] = √(Ka[HA])) and state the assumption. Convert mol cm⁻³ to mol dm⁻³ by multiplying by 1000 before logging, and check the answer lies in a sensible range (0–14).
Drawing structural formulae with carbon showing three bonds, or writing displayed formulae in which hydrogen is bonded to another hydrogen or to a halogen rather than to carbon, in organic structures.
Candidates copy condensed formulae without expanding them, and they do not apply the octet rule to carbon when drawing skeletal or displayed formulae under time pressure.
What to do instead
Before moving on, count bonds at every carbon: it must have four. When expanding a condensed formula, attach each hydrogen to the carbon it belongs to, and redraw any structure where two hydrogens or a halogen appear bonded to each other.
Naming or classifying an organic compound from its functional group alone, e.g. calling CH3CH2COOH an ester or naming CH3CH(OH)CH3 as propan-1-ol, without applying IUPAC numbering from the end nearest the principal functional group.
Functional groups are recognised but the priority and numbering rules are not applied systematically, and candidates do not check which end of the chain gives the lowest locant.
What to do instead
Use a fixed sequence: identify the principal functional group and its suffix, find the longest chain containing it, number from the end giving the lowest locant to that group, then name substituents alphabetically with locants. Practise on a short set such as propan-2-ol, butanoic acid and ethyl ethanoate until the routine is automatic.