HESI A2 Chemistry common mistakes
HESI A2 Chemistry · Understand the traps and how to avoid them.
When converting a word problem such as 'a 250 mL sample of 0.40 M NaCl,' the candidate plugs the numbers straight into the molarity equation and reports moles as 250 × 0.40 = 100 mol, without first converting milliliters to liters.
The candidate memorized M = mol/L as a formula but treats the units as decoration. Volume almost always appears in mL on the HESI A2, and the candidate has practiced with L so often that the conversion step becomes invisible under time pressure.
What to do instead
Before any calculation, write the given quantities with their units and circle anything not in base units (mL, mg, °C). Convert first, then substitute. Practice by rewriting M = mol/L as mol = M × L and physically crossing out 'mL' and writing '× 10⁻³ L' in its place. A habit check: if your answer to a solution problem is a three-digit number of moles, you skipped a conversion.
The candidate reads a density or molar-mass value off the wrong substance or rounds it aggressively (for example, using 23 g/mol for NaCl instead of about 58.5 g/mol), then carries the error through the whole stoichiometry chain.
Under time pressure, candidates grab the first atomic mass they remember (often the metal cation) and skip the periodic table lookup for the anion. Rounding feels efficient, but small molar-mass errors compound when multiplied by coefficients and by Avogadro's number.
What to do instead
Compute molar mass explicitly by writing each element, its count, and its atomic mass in a small table before doing anything else. Keep at least one decimal place for each atomic mass and round only the final answer. Build a short memorized list of the most common molar masses (H₂O 18.0, NaCl 58.5, CO₂ 44.0, NaOH 40.0, CaCO₃ 100.1, glucose 180.2) so you can sanity-check your computed value.
The candidate treats 'significant figures' as a rule to apply only at the end, writing 0.5234 g when the given mass was 0.52 g, or conversely dropping all precision and reporting 0.5 g.
Candidates learn sig-fig rules as abstract counting exercises and never connect them to the physical meaning of measurement precision. On a multiple-choice exam, the distractor answers are often built around wrong sig-fig choices, so the candidate cannot use the answer choices to recover.
What to do instead
Anchor the rule to the instrument: 'the last digit is my uncertainty.' Before answering, count sig figs in each given value and identify the least precise one; that sets the answer's precision. Practice with mixed operations: in 12.34 × 0.50, the answer has two sig figs (6.2), not four. Also practice scientific notation, since answers are often presented that way to test this exact skill.
The candidate counts electrons in a neutral atom's highest shell but then fails to adjust for ion charge, reporting Cl⁻ as having 17 electrons or Mg²⁺ as having 12.
The candidate memorized 'atomic number = electrons' for neutral atoms and never internalized that ions gain or lose electrons while the proton count (and element identity) stays fixed. The periodic table's group number reinforces the neutral-atom assumption.
What to do instead
Write a mini ledger for every ion question: protons = atomic number (never changes); electrons = protons minus charge. For Cl⁻: 17 − (−1) = 18. For Mg²⁺: 12 − (+2) = 10. Then state the electron configuration explicitly and check it against the nearest noble gas. Practicing this ledger for cations and anions from groups 1, 2, 16, and 17 builds the reflex.
The candidate identifies a period-3 element as having electrons in the 3d subshell, or writes the configuration of K as ending in 3d¹ instead of 4s¹.
The candidate applies the 'fill in order 1s 2s 2p 3s 3p 3d' pattern without the energy-ordering rule, and the periodic table's left-to-right layout invites assuming row 3 fills 3d. The 4s-before-3d exception is genuinely counterintuitive.
What to do instead
Use the diagonal (Aufbau) diagram and practice the sequence 1s 2s 2p 3s 3p 4s 3d 4p 5s 4d 5p. Remember that period 3 has no d electrons because the 3d subshell is higher in energy than 4s. Verify by checking the element's group and block on the periodic table: s-block and p-block positions tell you the last subshell directly.
The candidate predicts the trend in atomic radius or ionization energy by treating 'bigger atomic number' as 'bigger atom,' so they claim F is larger than Li or that ionization energy decreases across a period.
Candidates conflate atomic number with size and never separate the two competing effects: adding protons pulls electrons inward, while adding shells pushes them outward. Without explicitly naming which effect dominates, the trend is guesswork.
What to do instead
State the two competing effects out loud for every trend question. Across a period (left to right), protons increase while shell number stays the same, so radius decreases and ionization energy increases. Down a group, a new shell is added, so radius increases and ionization energy decreases. Anchor with extremes: smallest radius and highest ionization energy is He (or F among common elements); largest is Fr.
The candidate writes the formula for aluminum oxide as AlO or the formula for calcium nitrate as CaNO₃, ignoring the charges on the ions.
The candidate treats formula writing as stringing symbols together rather than as charge balancing. Polyatomic ions are especially fragile because the candidate may not know nitrate carries a −1 charge or that parentheses are needed when more than one is used.
What to do instead
For every ionic formula, write the cation charge and anion charge above the symbols first, then crisscross to balance: Al³⁺ and O²⁻ give Al₂O₃; Ca²⁺ and NO₃⁻ give Ca(NO₃)₂. Memorize the common polyatomic ions and their charges (nitrate −1, sulfate −2, carbonate −2, phosphate −3, hydroxide −1, ammonium +1). Always reduce the subscripts to the lowest whole-number ratio, and never write a subscript of 1.
The candidate names a covalent compound such as N₂O₄ as 'dinitrogen tetroxide' but names an ionic compound like FeCl₃ as 'iron trichloride,' mixing the two naming systems.
The candidate learned both systems but never built the decision rule that separates them. The prefix system (mono, di, tri) applies only to binary covalent compounds; ionic compounds use charge-based naming with Roman numerals for variable-charge metals.
What to do instead
First classify the compound: metal + nonmetal means ionic; two nonmetals means covalent. For covalent, use prefixes and drop 'mono-' on the first element (N₂O₄ = dinitrogen tetroxide; CO = carbon monoxide). For ionic, name the cation, then the anion with '-ide,' and use a Roman numeral only when the metal has variable charge (FeCl₃ = iron(III) chloride; FeCl₂ = iron(II) chloride). Group 1 and 2 metals and Al never need Roman numerals.
The candidate balances an equation by changing a subscript (turning H₂O into H₂O₂ to fix the hydrogen count) instead of changing a coefficient.
The candidate's goal becomes 'make the numbers match' rather than 'conserve atoms without changing substance identity.' Subscripts feel like the natural lever because they sit right next to the element symbol.
What to do instead
Adopt a hard rule: only coefficients in front of formulas may change. Balance in a fixed order: metals, then nonmetals, then hydrogen, then oxygen last. After balancing, do a full atom audit for each element on both sides, and check that all coefficients are in the lowest whole-number ratio. If a subscript change seems necessary, the formula is wrong or you have misread the reaction.
The candidate identifies a reaction type by the appearance of the reactants rather than by the products, labeling any reaction with O₂ as combustion even when the product is not CO₂ and H₂O, or labeling a single-replacement reaction as double replacement.
Reaction classification is often taught with example lists rather than a decision tree, so the candidate pattern-matches on surface features. Combustion, synthesis, and single replacement can all involve a pure element on one side, which blurs the categories.
What to do instead
Classify by the products and by the reactant pattern: A + B gives one product (synthesis); one reactant gives two or more products (decomposition); element + compound gives new element + new compound (single replacement); two compounds exchange partners (double replacement); hydrocarbon or C/H compound + O₂ gives CO₂ + H₂O (combustion). Write the products first, then assign the category from what you wrote, not from the reactants alone.