CLEP Chemistry common mistakes
CLEP Chemistry · Understand the traps and how to avoid them.
Balancing an equation by changing subscripts in a formula (writing H2O2 as H2O to make atoms match) instead of adjusting coefficients.
The candidate treats balancing as an atom-counting puzzle and reaches for the fastest symbol edit, forgetting that subscripts define the identity of the substance.
What to do instead
Before balancing, write each species' formula in a fixed box and practice the rule: only whole numbers in front of formulas may change. Check by naming each compound after balancing - if the name changed, the edit was illegal.
Treating a strong acid as fully dissociated in every context and computing pH from concentration without checking whether the acid is weak, or ignoring the autoionization of water for very dilute strong acids.
Textbook problems overwhelmingly feature strong acids, so the candidate pattern-matches 'acid means complete dissociation' and skips the Ka check.
What to do instead
Adopt a two-question gate: (1) Is it on the strong-acid list? (2) Is concentration below ~1e-6 M? If either answer is no, set up an ICE table with Ka or include water autoionization.
Reporting a rate or equilibrium answer with the wrong units or with moles instead of molarity when volume changes, e.g., adding solvent and assuming the amount of solute changes.
Candidates track 'how much stuff' rather than concentration, and dilution problems invite the intuition that adding water removes solute.
What to do instead
Write a one-line inventory before calculating: moles (conserved on dilution) versus molarity (changes). For kinetics and equilibrium, always convert to mol/L before substituting into rate laws or K expressions.
Sign errors in thermodynamics: computing ΔH for a reversed reaction without flipping the sign, or multiplying ΔH by a coefficient without scaling it.
Hess's law manipulation is mechanical, and candidates focus on summing numbers while forgetting that reversing a reaction reverses the enthalpy change.
What to do instead
Use a table with columns for reaction, multiplier, and sign flip, and recompute the target reaction from scratch. Verify the final sign against physical intuition (combustion should be exothermic, ΔH negative).
Assuming a reaction goes to completion and using the limiting reactant to compute product mass even when the problem describes an equilibrium or a reversible process.
Stoichiometry drills train 'limiting reactant then convert to grams,' and candidates apply that template to equilibrium scenarios without noticing the word 'equilibrium' or a K value.
What to do instead
Before any stoichiometric calculation, scan for K, Ka, Ksp, or 'at equilibrium.' If present, use an ICE table and solve with the equilibrium constant; reserve limiting-reactant logic for reactions explicitly driven to completion.
Confusing oxidation and reduction, or assigning oxidation numbers by electronegativity intuition rather than by the standard rules (e.g., giving oxygen +2 in a peroxide).
The mnemonic 'OIL RIG' is memorized but not tied to electron transfer, and the peroxide and hydride exceptions are rarely practiced.
What to do instead
Assign oxidation numbers systematically using the ordered rule list (free elements 0, group 1 +1, oxygen usually -2 except peroxides, hydrogen usually +1 except hydrides) and then compute the change per atom to identify what is oxidized and reduced.
Predicting molecular geometry or polarity from the number of electron domains without subtracting lone pairs, so H2O is called linear or CO2 is called bent.
Candidates count bonds only and forget that lone pairs occupy domains and compress bond angles.
What to do instead
Use a fixed two-step procedure: count electron domains (bonds plus lone pairs) to get the electron geometry, then remove lone pairs to get the molecular shape. Finish by checking symmetry of the resulting shape for polarity.
Applying Le Chatelier's principle by shifting equilibrium in response to a change in pressure caused by adding an inert gas at constant volume, or shifting when a catalyst is added.
Candidates memorize 'pressure increase shifts to fewer moles' without distinguishing partial pressure changes from total pressure changes.
What to do instead
Classify the disturbance first: does it change the partial pressure or concentration of a reacting species? Inert gas at constant volume changes neither, and a catalyst changes neither - so no shift. Only changes to reacting species' concentrations or partial pressures shift the position.
Mixing up the gas laws: using Charles's law for a constant-temperature problem or plugging Celsius temperatures directly into the ideal gas law.
The law names blur together under time pressure, and Celsius-versus-Kelvin is a habitual omission.
What to do instead
Before substituting, write which variables are held constant (P, V, T, n) and select the law that matches. Convert all temperatures to kelvin as the first line of every gas calculation, and sanity-check that volume scales with kelvin, not Celsius.
Reading a titration or calorimetry result from a burette or thermometer without accounting for the meniscus, parallax, or the heat capacity of the calorimeter, and reporting more significant figures than the glassware supports.
Experimental questions are treated as pure calculation, so candidates ignore measurement conventions and instrument precision.
What to do instead
Practice reading burettes at eye level from the bottom of the meniscus to the nearest 0.01 mL, and always include the calorimeter's heat capacity in q = mcΔT + CΔT. Round final answers to the precision of the least precise measurement.