PSEB SS Chemistry common mistakes
PSEB SS Chemistry · Understand the traps and how to avoid them.
Writing the balanced equation with correct formulas but leaving out state symbols, or assigning (aq) to a weak acid that is actually written in molecular form, so the ionic equation that follows is inconsistent with the molecular one.
Candidates treat state symbols as decoration and memorise 'ionic equations' as a separate topic, so they never check that the species they cancel actually exist as ions in the given conditions.
What to do instead
Before writing the net ionic equation, build a three-line table: molecular, complete ionic (every soluble strong electrolyte split), net ionic. Mark each species as strong electrolyte (split) or weak/non-electrolyte (keep whole) using a short checklist: strong acids, strong bases, soluble salts. Then cancel spectator ions and verify charge and atom balance on the final line.
Calculating moles with n = m/M using a molar mass assembled from the wrong formula, e.g. using the empirical formula mass instead of the molecular formula mass, or forgetting that a hydrated salt's molar mass includes the water of crystallisation.
Under time pressure candidates grab the simplest formula visible in the question stem and skip the step of establishing which formula the mass refers to; hydration is memorised as a topic but not linked to routine mole calculations.
What to do instead
Adopt a fixed opening ritual for every quantitative item: write the species with its full formula including any dot-water, compute M as a separate labelled line, then substitute. For empirical/molecular problems, explicitly compute empirical mass, divide given molar mass by it, and state the multiplier before writing the molecular formula.
Applying Le Chatelier predictions that ignore the effect of a catalyst or of adding an inert gas at constant volume, and stating that a catalyst shifts equilibrium to the products.
The phrase 'affects the equilibrium' is over-generalised from the rate chapter; candidates do not distinguish between factors that change the position of equilibrium and factors that only change how fast it is reached.
What to do instead
Classify each disturbance in a two-column table: 'changes K or position' versus 'changes rate only'. Catalysts and inert gas at constant volume go in the second column with the justification that forward and reverse rates are affected equally or partial pressures are unchanged. Practise by predicting the effect on both rate and yield for the same disturbance.
Drawing a Lewis structure or predicting molecular shape without first counting electron pairs around the central atom, then declaring a molecule non-polar because it contains polar bonds.
VSEPR and polarity are learned as separate definitions; candidates jump straight to a remembered shape (e.g. 'water is bent') without deriving it, and conflate bond polarity with molecular polarity.
What to do instead
Use a fixed four-step routine: total valence electrons, electron-pair geometry, molecular geometry after lone-pair placement, then vector sum of bond dipoles. For polarity, always sketch the 3D arrangement and check for symmetry; state explicitly whether lone pairs or identical substituents cancel the dipoles.
Writing half-equations for redox in acidic or alkaline medium without adding H2O and H+/OH- to balance O and H, so the final equation balances atoms of the main elements but not charge.
Candidates balance only the element that changes oxidation state and treat H and O as automatically balanced; the medium (acidic vs alkaline) is often overlooked entirely.
What to do instead
Follow the standard sequence every time: balance the atom changing oxidation state, balance O with H2O, balance H with H+, balance charge with electrons, then (if alkaline) neutralise H+ by adding OH- to both sides and cancel water. Finish by checking total charge on each side equals.
In titration calculations, using the burette reading or the average titre directly as moles of analyte, or averaging titre values that include an obvious outlier (e.g. a rough titration).
The calculation is memorised as a formula chain without the conceptual step that concentration must be multiplied by volume in dm3, and candidates are reluctant to discard data because discarding feels like losing information.
What to do instead
Write the stoichiometric ratio from the balanced equation before any arithmetic, convert all volumes to dm3 explicitly, and show the mole ratio line. For titre selection, state the criterion (concordant values within about 0.10 cm3), exclude the rough titre, and record the accepted mean in the answer.
Reporting a titration result to the precision of a calculator display (e.g. 0.10237 mol dm-3) or omitting units and the correct number of significant figures derived from the burette readings.
Candidates equate 'more digits' with 'more accurate' and are not taught that the apparatus precision sets the reportable precision; mark schemes reward consistent significant figures.
What to do instead
Before writing the final answer, note the precision of each measurement (burette to 0.05 cm3, pipette to 0.1 cm3, balance to 0.001 g) and round the result to the least precise input. Always attach units and state the value as a mean of concordant titres.
Describing a qualitative salt analysis by naming the reagent and the colour but not the sequence of tests, so a positive result for one ion is claimed before interfering ions have been removed.
Tests are memorised as isolated colour facts rather than as a logical flow; candidates do not appreciate that a precipitate forming in the first test may mask a later one.
What to do instead
Learn each group as an ordered decision tree: preliminary tests, then group separation, then confirmatory test for the specific ion. For every claim, state the observation, the inference, and the confirmatory step (e.g. flame colour, gas evolved on heating, precipitate soluble in excess reagent).
In enthalpy of neutralisation or displacement experiments, using the mass of the solute rather than the total mass of solution in q = mcΔT, or ignoring heat loss to the calorimeter and thermometer.
The formula is applied mechanically with whatever mass is prominent in the question, and the assumption that the experiment is adiabatic is never examined.
What to do instead
State the assumption explicitly before calculating: total mass of solution, specific heat capacity of the solution, no heat loss. Compute q, then divide by moles of limiting reactant to get ΔH per mole, and comment on the direction of heat loss and its effect on the magnitude of the measured value.
Drawing apparatus for distillation, filtration or gas preparation without clamping, without a delivery tube correctly placed, or with the thermometer bulb in the liquid during distillation.
Diagrams are copied from memory as pictures rather than as functional assemblies; candidates do not ask what each component is doing, so placement errors go unnoticed.
What to do instead
Before drawing, state the purpose of each part in one sentence (what it separates, what it seals, what it measures). Then check four points: support/clamping, correct joint connections, thermometer bulb position relative to the vapour, and open vs closed system for gas collection. Label every part you rely on for marks.