NSSCO Chemistry (6117) common mistakes
NSSCO Chemistry (6117) · Understand the traps and how to avoid them.
Calculates moles by dividing mass by the relative atomic mass of the compound rather than its relative formula mass, or converts cm³ to dm³ by multiplying by 1000.
Formula triangles are memorised without unit reasoning; Mr is a ratio with no units, while burette and measuring-cylinder readings are in cm³.
What to do instead
Write n = m/Mr and check units: g ÷ g mol⁻¹ = mol. For solutions write n = c × V(dm³), converting V(cm³)/1000 first. Sanity-check: 25 cm³ = 0.025 dm³, so 0.1 mol dm⁻³ contains 0.0025 mol.
Draws ionic dot-and-cross diagrams with shared electron pairs or without square brackets and charges, for example showing NaCl as a molecule with a shared pair.
Covalent and ionic models blur; candidates reproduce a shell picture without deciding whether electrons are transferred or shared.
What to do instead
State the type before drawing: metal + non-metal means transfer. Show outer shells only, use a different symbol for each atom’s electrons, add an arrow for transfer, then draw each ion in square brackets with its charge and a full outer shell (Na⁺ [2,8], Cl⁻ [2,8,8]). Do not join ions with a line.
Chooses evaporation to dryness for a salt that decomposes on heating, or uses filtration to separate two dissolved solids.
Keyword matching: “separate salt and water” suggests evaporation, but the method must fit the salt’s thermal stability and whether the solid is dissolved.
What to do instead
Use a decision routine: insoluble solid + liquid → filtration; dissolved solid → crystallisation, or evaporation only if thermally stable; miscible liquids → fractional distillation; solvent recovery → simple distillation. Check solubility and stability before naming the method.
In a titration calculation, ignores the stoichiometric ratio and sets c₁V₁ = c₂V₂ for a reaction such as 2NaOH + H₂SO₄, or includes the rough titre in the mean.
The cV shortcut is valid only for 1:1 reactions; the rough titration is deliberately approximate and should be excluded from the mean.
What to do instead
Balance the equation first, calculate moles of the solution with known concentration, apply the mole ratio, then divide by the other volume. Use only concordant titres within 0.10 cm³ for the mean and quote burette readings to 0.05 cm³.
Gives an ion’s electron configuration by adding or removing electrons from an inner shell, for example Cl⁻ as 2,8,7 or Mg²⁺ as 2,8,2, and writes Fe²⁺ as 2,8,8.
Shell rules are learned for neutral atoms; the charge is not linked to total electron count, and transition-metal exceptions are unfamiliar.
What to do instead
Check total electrons = protons − charge. For main-group ions, adjust the outer shell to the nearest noble-gas configuration (Cl⁻ 2,8,8; Mg²⁺ 2,8). For transition-metal ions, remove 4s electrons before 3d (Fe²⁺ 2,8,14; Fe³⁺ 2,8,13) and learn common ions.
Explains Group I reactivity by saying atoms “get bigger so they attract electrons more strongly”, or says Group VII reactivity increases down the group.
Opposite trends are memorised as slogans without the electron-transfer model; distance and shielding are not linked to attraction.
What to do instead
Write the trend with its cause: down Group I, the outer electron is further from the nucleus and more shielded, so it is lost more easily and reactivity increases. Down Group VII, an incoming electron is attracted less strongly because of greater distance and shielding, so reactivity decreases. Compare Li/Na/K and F/Cl/Br.
Claims a catalyst increases the yield of a reversible reaction, or that raising temperature always increases yield because “particles collide more”.
Rate and equilibrium position are conflated; a catalyst speeds both directions equally, while temperature changes the equilibrium constant.
What to do instead
Separate the ideas: rate is how fast equilibrium is reached; yield is the equilibrium position. A catalyst lowers activation energy and does not change yield. For temperature, use the sign of ΔH: heating favours the endothermic direction, cooling favours the exothermic direction.
Draws an addition polymer repeat unit still containing the C=C double bond, or names CH₃CH=CH₂ as propane.
Polymerisation is memorised as a picture rather than as “open the double bond”, and organic naming suffixes are overlooked.
What to do instead
For addition polymerisation, draw the monomer, change C=C to C–C, put brackets around the two backbone carbons with continuation bonds, and keep substituents on the correct carbon. For naming, find the longest chain containing the double bond and use -ene; check hydrogens against the alkane formula.
Identifies carbon dioxide with a burning-splint “pop”, or says sulfur dioxide causes ozone depletion rather than acid rain.
Gas tests and environmental effects are learned as isolated facts, so similar-sounding pollution terms get swapped.
What to do instead
Attach each test to its observation: CO₂ turns limewater milky; H₂ gives a squeaky pop with a lighted splint; O₂ relights a glowing splint; Cl₂ bleaches damp litmus. Link gases to effects: SO₂ and NOₓ → acid rain; CO₂ and CH₄ → greenhouse effect; CFCs → ozone depletion.
Writes “human error” or “not accurate” as a source of error, or states a conclusion without quoting the data trend.
Practical reports reward a conclusion, but candidates are not taught to separate random, systematic and procedural error.
What to do instead
Name the specific cause and its effect: “heat loss to surroundings lowered the maximum temperature, so ΔT is too small”; “the burette was not rinsed with the solution, so the titre is too large”. In conclusions, cite two readings and the trend, then state whether the hypothesis is supported.