ORELA Chemistry (NT321) common mistakes
ORELA Chemistry (NT321) · Understand the traps and how to avoid them.
When balancing a redox equation in acidic or basic solution, the candidate balances atoms and charge but never adds H2O or OH- to the half-reactions, then cannot reconcile the electron count with the final equation.
The candidate memorized the 'balance atoms, then charge' slogan from a general chemistry course but never practiced the full half-reaction procedure in basic medium, where O and H must be supplied from water and hydroxide rather than assumed to be present in the given species.
What to do instead
Drill the fixed half-reaction algorithm on paper: separate into half-reactions, balance all elements except O and H, add H2O to balance O, add H+ to balance H, add electrons to balance charge, then for basic solution add OH- to both sides equal to the H+ count and combine water. Verify by checking that atoms and total charge match on both sides before combining half-reactions and multiplying by the least common multiple of electrons.
The candidate treats the ideal gas law as universally applicable and plugs values into PV = nRT for a gas at high pressure or near its boiling point, ignoring compressibility and intermolecular forces.
Ideal-gas problems dominate introductory coursework, so the candidate develops a reflex of reaching for PV = nRT whenever pressure, volume, and temperature appear, without checking whether the conditions fall outside the ideal regime.
What to do instead
Before using PV = nRT, check the conditions: pressures above roughly 10 atm or temperatures near the substance's boiling point signal non-ideal behavior. Practice deciding when to use van der Waals' equation with its a and b constants, and when a real-gas or phase consideration is needed. Build a habit of estimating the molar volume at STP (about 22.4 L) and comparing it to the calculated value to flag implausible answers.
When asked to identify the limiting reactant, the candidate compares the given masses or moles of the reactants directly rather than converting each to moles of product using the stoichiometric coefficients.
The candidate conflates 'smallest amount present' with 'limiting,' a shortcut that happens to work in some textbook problems but fails whenever the coefficients differ from 1:1 or the reactants are supplied in different units.
What to do instead
Always divide the moles of each reactant by its coefficient in the balanced equation, then compare those quotients; the smallest quotient identifies the limiting reactant. Follow with a mass-of-product calculation from that reactant and check that the required amount of the other reactant does not exceed what is available. Practice with at least one problem where the visually smaller quantity is not limiting.
The candidate predicts molecular geometry from the number of bonded atoms alone, calling a molecule with lone pairs on the central atom linear or trigonal planar when the electron-domain geometry is tetrahedral.
Introductory courses often emphasize VSEPR shapes of simple molecules without lone pairs, so the candidate under-weights lone-pair repulsion and forgets that lone pairs occupy domains and compress bond angles.
What to do instead
Use a two-step method: first count all electron domains (bonds plus lone pairs) to get the electron-domain geometry, then remove lone pairs to get the molecular geometry. Memorize the standard deviations, such as water's bent shape with an angle under 109.5 degrees and ammonia's trigonal pyramidal shape. Practice drawing Lewis structures and labeling each domain before naming the shape.
The candidate applies the octet rule rigidly and cannot draw acceptable structures for electron-deficient species such as BF3 or for hypervalent species such as SF6 and PCl5.
Most practice structures in early coursework obey the octet rule, so the candidate treats it as a law rather than a guideline and becomes stuck when a valid structure requires fewer than eight or more than eight electrons on the central atom.
What to do instead
Learn the exceptions explicitly: beryllium and boron compounds are often electron-deficient, and elements in period 3 and below can expand their octets using d orbitals or three-center bonding. Practice drawing BF3, SF6, and PCl5, then verify formal charges to confirm the structures are reasonable. Also practice recognizing when an expanded octet is not permissible, as with second-period elements like carbon and nitrogen.
The candidate confuses intermolecular forces with intramolecular bonds and predicts boiling points or solubilities by citing covalent bonds within molecules instead of the forces between them.
The terms 'bond' and 'force' are used loosely in everyday language, and the candidate has not internalized that phase changes and solubility depend on the strength of attractions between separate molecules.
What to do instead
Practice ranking substances by boiling point and explicitly naming the dominant intermolecular force in each: London dispersion, dipole-dipole, or hydrogen bonding. State the rule that stronger intermolecular forces raise boiling points and that hydrogen bonding requires H attached to N, O, or F. For solubility, compare the polarity of solute and solvent and identify the specific interaction that stabilizes the solution.
When solving equilibrium problems, the candidate omits the change row of an ICE table or assumes the change equals the initial concentration, producing a wrong equilibrium constant or wrong final concentrations.
The candidate has seen many problems where the reaction is assumed to go essentially to completion and carries that habit into genuine equilibrium problems, where a small but nonzero change must be computed and often solved via a quadratic or approximation.
What to do instead
Set up the full ICE table every time: initial, change, equilibrium rows, with the change expressed in terms of an unknown x and scaled by the stoichiometric coefficients. Then write the equilibrium expression in terms of x and solve. Check whether the small-x approximation is justified by comparing x to the initial concentration (typically under 5 percent) and verify the final values by substituting back into the equilibrium expression.
The candidate treats a strong acid and a weak acid of equal concentration as having the same pH, or calculates pH from concentration without considering the degree of dissociation.
The candidate memorizes pH = -log[H+] and applies it directly to the nominal acid concentration, forgetting that weak acids dissociate only partially and that the dissociation constant governs the actual hydrogen ion concentration.
What to do instead
For weak acids, set up the equilibrium expression with Ka, let x equal the hydrogen ion concentration, and solve, checking the small-x approximation. Compare results for a strong acid and a weak acid at the same concentration to see the pH difference. Also practice calculating pH at the half-equivalence and equivalence points of a titration, where the relevant species change.
The candidate confuses oxidation and reduction or misassigns oxidation numbers, particularly in polyatomic ions and in compounds containing oxygen or hydrogen.
The mnemonic 'OIL RIG' covers electron transfer but not oxidation-state bookkeeping, and the candidate has not practiced the priority rules for assigning oxidation numbers when oxygen, hydrogen, and halogens appear together.
What to do instead
Memorize the oxidation-number rules in priority order: elements in their standard state are zero, monatomic ions equal their charge, oxygen is usually -2 (except in peroxides and with fluorine), hydrogen is usually +1 (except in metal hydrides), and the sum equals the overall charge. Practice assigning numbers in species like permanganate, dichromate, and hydrogen peroxide, and identify which atom is oxidized and which is reduced in each reaction.
The candidate draws curved arrows for a reaction mechanism starting from the product or from the wrong atom, and cannot distinguish a nucleophile from an electrophile in the given step.
Mechanism problems require tracking electron pairs and charge, a skill distinct from memorizing reaction types, and the candidate often reproduces a remembered arrow pattern without checking which species is electron-rich and which is electron-poor.
What to do instead
Before drawing any arrow, label each species as nucleophile or electrophile and identify the electron-rich site and the electron-poor site. Draw arrows from the electron pair source to the electron-poor atom, and after each step check that formal charges and the octet count are consistent. Practice with substitution and addition reactions, verifying that each intermediate is chemically reasonable.
The candidate reads a laboratory procedure and reports the calculated result without considering significant figures, unit conversions, or the precision of the measuring instruments used.
The candidate treats calculations as pure arithmetic and carries calculator output to many decimal places, ignoring that the reported answer must reflect the least precise measurement and that instruments such as graduated cylinders and analytical balances have different precision.
What to do instead
Before calculating, record the precision of each instrument and apply the significant-figure rules for multiplication and addition at the end. Practice unit conversions with dimensional analysis, canceling units explicitly. When reporting a result, round to the appropriate number of significant figures and state the uncertainty, and check whether the answer is physically plausible given the quantities measured.
The candidate describes safety procedures generically, such as 'wear goggles and be careful,' instead of naming the specific hazard, the specific precaution, and the correct response to a spill or exposure for the substance involved.
Safety content is often learned as a checklist at the start of a lab course and never revisited, so the candidate cannot connect a specific reagent, such as a strong acid, a flammable solvent, or a toxic gas, to its handling requirements.
What to do instead
Build a reagent-by-reagent safety table: hazard class, required personal protective equipment, ventilation needs, storage incompatibilities, and spill response. Practice matching specific scenarios, such as a mercury spill or an acid splash, to the correct procedure and waste-disposal route. Review Safety Data Sheet sections for the reagents most likely to appear in a school laboratory.
The candidate writes a lab conclusion that restates the procedure and the numerical result but does not evaluate sources of error or explain whether the hypothesis was supported.
The candidate treats the lab report as a record of what was done rather than an argument about what the data mean, often because earlier coursework rewarded completion over analysis.
What to do instead
Structure conclusions around three elements: the claim, the evidence from the data, and the reasoning that links them. Identify at least two specific sources of error, classify each as systematic or random, and state how each would shift the result. Compare the experimental value to the accepted value, calculate percent error, and propose a concrete procedural change that would reduce the largest error source.