ICO Part B (ORI) common mistakes
ICO Part B (ORI) · Understand the traps and how to avoid them.
Reporting a titration result to four decimal places from a burette read to ±0.05 cm³ and a 0.1 mol dm⁻³ titrant, without propagating the volume and concentration uncertainties into the final molarity.
Candidates treat significant figures as a formatting rule learned in school rather than a consequence of instrument precision; the burette, pipette and balance each contribute independent uncertainty that must be combined, and the endpoint detection adds a further term.
What to do instead
Before writing the answer, list each measured quantity with its absolute uncertainty (burette ±0.05 cm³ per reading, Class A pipette ±0.03 cm³, analytical balance ±0.0001 g), convert to relative uncertainties, combine in quadrature for the final quantity, and quote the result as value ± expanded uncertainty with the correct number of significant figures.
Drawing a straight line through all kinetic or calibration data points by eye or by joining successive points, then reading a rate constant or unknown concentration from that line.
Candidates confuse 'line of best fit' with 'connect the dots' and have not been taught that least-squares regression minimises residuals in the y-direction and that outliers should be examined, not silently averaged in.
What to do instead
Plot the raw points, inspect for curvature or outliers, then fit y = mx + c by least squares (calculator or spreadsheet), report the slope with its standard error, and state R². For first-order kinetics, plot ln[A] vs t and confirm linearity before extracting k from the slope.
Using a pH meter or glass electrode without calibrating against at least two buffers bracketing the expected value, and stirring the solution while the electrode stabilises.
Candidates believe the meter 'just reads pH' and are unaware of the Nernstian slope drift, asymmetry potential and junction potential that make a single-point calibration unreliable; stirring is copied from titrations without realising it destabilises the reading.
What to do instead
Calibrate with two buffers (e.g. pH 4.00 and 7.00, or 7.00 and 10.00) spanning the sample, record the slope (should be 95–105 % of theoretical), measure with gentle or no stirring, allow the reading to stabilise, and rinse the electrode between samples.
Recording an IR or UV–vis spectrum of a sample that is too concentrated, giving a flat-topped or off-scale peak, and reporting the λmax or band position from that saturated trace.
Candidates assume a strong signal is a good signal; they do not know that absorbance above ~1.5 leaves the linear range of the Beer–Lambert plot and that saturated IR bands distort peak shape and position.
What to do instead
Dilute until the maximum absorbance falls between 0.1 and 1.0 (ideally 0.2–0.8), re-record, and confirm linearity by measuring at two concentrations. For IR, use a thin film or KBr disc thin enough that the strongest band is not clipped.
Assigning a structure from ¹H NMR alone, ignoring integration ratios, coupling patterns and the possibility of exchangeable protons, then presenting the assignment as conclusive.
Candidates over-rely on chemical shift tables and do not cross-check with IR, mass spectrometry and elemental analysis; they forget that OH/NH protons may be broad, exchange with D₂O, or be absent, and that integration must be normalised to the smallest whole-number ratio.
What to do instead
Build the assignment from at least three independent techniques: exact mass or M⁺/isotope pattern for the molecular formula, IR for functional groups, ¹H and ¹³C NMR for connectivity and symmetry, and elemental analysis for purity. State which data are inconsistent or ambiguous rather than forcing a fit.
Reporting a melting point as a single temperature without the start and end of the range, or without stating the heating rate and whether the sample was dried.
Candidates see the melting point as a pass/fail purity check and do not appreciate that the range width and the rate of heating both carry information about purity and that a wet sample depresses and broadens the range.
What to do instead
Record the full range (e.g. 121–123 °C), the heating rate (typically 1–2 °C min⁻¹ near the melt), and the literature value with its source. A range wider than ~2 °C or depressed relative to literature indicates impurity or residual solvent and should be reported as such.
Running a reaction or work-up without a written risk assessment, and disposing of halogenated, heavy-metal or peroxide-forming waste down the sink or into the general organic waste.
Candidates treat safety paperwork as an administrative hurdle and have not internalised that waste streams are segregated by chemical class; they also underestimate peroxide formation in ethers and the incompatibility of oxidisers with organics.
What to do instead
Before starting, complete a risk assessment naming the hazards (GHS phrases), the controls, and the waste route for each reagent. Segregate halogenated from non-halogenated waste, collect heavy-metal waste separately, date and test peroxide-forming solvents, and never mix incompatible oxidisers and organics.
Calculating a gravimetric result from the weighed precipitate without checking that it was digested, filtered, washed free of soluble salts, and ignited to constant mass.
Candidates focus on the stoichiometry and forget that gravimetry is only as good as the precipitate; occlusion, peptisation and incomplete ignition all bias the result in one direction and are invisible in the arithmetic.
What to do instead
Digest the precipitate (stand hot), wash with an appropriate electrolyte solution, test the filtrate for the precipitating ion, and ignite or dry to constant mass (repeat until successive weighings agree within 0.2 mg). Report the gravimetric factor and the mass of the ignited form explicitly.
Quoting a yield or a rate constant from a single run, or comparing two methods on single measurements, without replication or an estimate of repeatability.
Candidates are trained to produce one 'correct' number and have not been shown that analytical and kinetic results have a distribution; without replicates there is no way to distinguish a real difference from run-to-run variation.
What to do instead
Perform at least triplicate measurements for key quantities, report the mean and standard deviation (or standard error), and use a t-test or F-test before claiming a difference between methods or conditions. State n and the confidence level.
Writing a coordination compound's formula and name without specifying oxidation state, coordination number and geometry, or omitting the counter-ions and hydration.
Candidates memorise naming rules but do not connect the name to the structural information (ligand charge, metal oxidation state, coordination geometry) that the name must encode, and they forget that lattice water and counter-ions change the formula mass and the analysis.
What to do instead
For every complex, state the metal oxidation state, the coordination number and geometry, the ligand charges, and the counter-ions including water of crystallisation. Check the name against the formula by summing charges, and confirm the formulation with elemental analysis, molar conductance and magnetic moment where relevant.