CSET Science: Chemistry common mistakes
CSET Science: Chemistry · Understand the traps and how to avoid them.
When a stoichiometry item gives masses of two reactants, the candidate computes the product mass from the reactant that appears first or is 'larger,' instead of testing both against the mole ratio to find the limiting reagent.
Under time pressure candidates default to a single familiar calculation path and skip the comparison step; many also conflate 'excess' with 'larger mass' rather than fewer moles relative to the ratio.
What to do instead
Adopt a fixed three-line routine for every limiting-reagent item: (1) convert each reactant to moles, (2) divide each by its coefficient and compare, (3) label the smaller quotient as limiting and compute products only from it. Drill ten items where the limiting reagent is the one with the larger mass, so the shortcut visibly fails.
The candidate treats balanced equations as mole-to-mass direct conversions, multiplying grams of A by the coefficient ratio to get grams of B.
Coefficients are learned as 'the numbers in front' without a firm attachment to particles/moles; the gram-to-mole gate is skipped because it feels like an extra step.
What to do instead
Write the unit path explicitly above each calculation (g A -> mol A -> mol B -> g B) and cancel units as if they were algebraic quantities. Practice with a problem where the molar masses are equal, so only correct unit tracking distinguishes right from wrong.
The candidate writes electron configurations or orbital diagrams that violate the Aufbau order or Hund's rule, such as pairing electrons in a p subshell before singly occupying all three orbitals, or placing 4s after 3d filling.
Rules are memorized as isolated slogans and the candidate never draws orbital boxes, so the violation is invisible in plain notation.
What to do instead
For every configuration item, draw the orbital box diagram first and derive the notation from it. Build a one-page energy-order ladder (1s 2s 2p 3s 3p 4s 3d 4p) and use it to answer anion/cation and transition-metal items, remembering that 4s electrons are removed before 3d for cations.
The candidate predicts periodic trends by atomic number alone, saying radius increases across a period or that ionization energy increases down a group, rather than reasoning from effective nuclear charge and shielding.
Trends are memorized as arrows on a table without the underlying Coulombic cause, so the arrows get reversed or misapplied to ions and isoelectronic species.
What to do instead
Re-derive each trend from 'more protons pulling on the same shell = smaller and harder to ionize.' For isoelectronic series, rank by nuclear charge (e.g., O2- > F- > Na+ > Mg2+ in radius). Answer comparison items by writing the proton count and shell count side by side.
The candidate treats strong versus weak acids and concentrated versus dilute as the same axis, predicting that a 0.1 M weak acid has low pH because it is 'weak,' or that a dilute strong acid is 'weak.'
Everyday meanings of 'strong' and 'weak' collide with the technical definitions; degree of ionization and analytical concentration are stored as one idea.
What to do instead
Maintain two separate columns in notes: extent of ionization (strong/weak) and amount per liter (concentrated/dilute). For weak-acid pH items, set up Ka = x^2/(C - x) and state the approximation explicitly. Compare a 0.1 M HCl with a 0.1 M acetic acid and a 1e-4 M HCl to see all four combinations.
The candidate balances redox half-reactions without adding water and H+ (or OH-) to the correct sides, or fails to multiply each half-reaction so electrons cancel before adding.
The atom and charge balance checks are skipped because the candidate stops as soon as the main elements look balanced.
What to do instead
Use a fixed sequence: balance non-O/H atoms, balance O with H2O, balance H with H+, balance charge with electrons, then scale halves to equal electrons and add. Verify total charge on both sides as a final line. For basic solution, add OH- to both sides to neutralize H+ afterward.
The candidate confuses the signs and definitions of enthalpy, entropy, and Gibbs free energy, calling an exothermic reaction spontaneous regardless of temperature, or misreading a negative delta-G as non-spontaneous.
Delta-H and delta-G are both 'energy' in casual memory, and the temperature dependence in delta-G = delta-H - T delta-S is not applied case by case.
What to do instead
Build a four-cell table of sign combinations (H+,S+; H-,S+; H+,S-; H-,S-) and state spontaneity and the temperature threshold for each. Practice items that ask for the crossover temperature T = delta-H/delta-S, and always write the sign convention next to the symbol before answering.
The candidate draws Lewis structures that exceed the octet on period-2 atoms, ignores formal charge when several valid skeletons exist, or omits resonance and lone-pair placement on the central atom.
The candidate stops at the first structure that 'looks connected' and does not run the formal-charge and octet checks that choose among candidates.
What to do instead
Apply a checklist to every structure: count valence electrons, place the least electronegative atom centrally, complete octets, then compute formal charge on each atom. Choose the skeleton with formal charges closest to zero. Mark equivalent resonance forms and note when expanded octets are allowed only for period 3 and below.
The candidate predicts molecular geometry from the number of bonded atoms only, calling a molecule with lone pairs on the central atom by its electron-domain name (e.g., calling water linear or ammonia trigonal planar).
VSEPR is remembered as 'count atoms' rather than 'count electron domains, then subtract lone-pair influence,' and the lone pairs are not drawn.
What to do instead
For each item, first write the Lewis structure with all lone pairs, then count domains (bonds + lone pairs) to get the electron geometry, then apply the lone-pair compression to get the molecular geometry and approximate bond angle. Memorize the domain-to-geometry table and practice the bent and trigonal pyramidal cases specifically.
The candidate answers gas-law items by plugging into PV = nRT with mixed units (mL with atm, Celsius with kelvin) or by assuming the identity of the gas does not matter when the item asks for a mass or molar mass.
Formula recall is strong but unit discipline and the distinction between amount and identity are weak; temperature conversions are skipped under time pressure.
What to do instead
Before every gas calculation, write a unit-conversion line (T in K, V in L, P in atm) and confirm R matches the units chosen. For molar-mass items, rearrange to n = PV/RT and then M = m/n. Practice combined-law items where the gas is changed, so the molar mass must be carried through.