NY Regents Chemistry common mistakes
NY Regents Chemistry · Understand the traps and how to avoid them.
Reporting an answer to the calculator's full display (e.g., 4.386666667 g) instead of rounding to the significant figures justified by the given data, and losing credit on constructed-response items.
Students treat the calculator output as the answer and were often never drilled on the Atlantic/Pacific significant-figure rules or on counting sig figs in measured values like 0.04050 g (4 sig figs).
What to do instead
Before computing, write the sig-fig count beside each given value; after computing, round the final answer to the smallest count. Practice the rule set: non-zero digits count, sandwiched zeros count, leading zeros never count, trailing zeros after a decimal count. On Regents constructed response, also check that the unit and correct number of decimal places are shown.
Classifying a change or substance incorrectly: calling dissolving, boiling, or crushing a chemical change, or labeling a mixture as a compound (or vice versa).
Students memorize 'new substance = chemical change' as a slogan but never apply it to borderline cases, and they confuse 'can be separated by physical means' (mixture) with 'fixed composition, broken down only chemically' (compound).
What to do instead
For each scenario, ask two questions in order: (1) Is a new substance with new properties formed? If no, it is physical. (2) Can it be separated by filtration, distillation, or evaporation? If yes, it is a mixture. Build a personal sort of 15 examples (salt water, dry ice subliming, rusting nail, electrolysis of water, air, brass) and justify each in one sentence.
Drawing a Bohr model or writing an electron configuration that violates the Aufbau order or the octet/duet limit, e.g., putting 8 electrons in the second shell before filling 2s and 2p correctly, or writing 1s2 2s2 2p6 3s2 3p6 4s2 3d1 for scandium as 3d before 4s.
Students memorize a jumbled configuration chart without understanding subshell energy ordering, and they conflate 'shell capacity 2n squared' with the order electrons actually fill.
What to do instead
Use the diagonal (Aufbau) diagram every time and check with the periodic table: s-block fills 2, p-block 6, d-block 10. For ions, remove electrons from the highest n shell first (4s before 3d for transition metals). Verify each configuration by summing superscripts to equal the atomic number.
Predicting periodic trends with the wrong reasoning, e.g., saying atomic radius increases across a period because 'more shells are added', or that ionization energy decreases across a period.
Students memorize trend arrows without the underlying cause (effective nuclear charge vs. shielding and shell number), so they invert trends when the question is phrased unusually.
What to do instead
Anchor every trend to two variables: number of shells (down a group) and effective nuclear charge / protons pulling on the same shell (across a period). State the cause in your scratch work before choosing. Check yourself: radius increases down and left; ionization energy and electronegativity increase up and right; metallic character increases down and left.
Treating all bonds as purely ionic or purely covalent and misclassifying compounds like HCl, CO2, or MgCl2, or drawing a Lewis structure with the wrong number of shared pairs and missing lone pairs.
Students use a binary 'metal plus nonmetal' shortcut without considering electronegativity difference, and they skip the electron-counting step when drawing Lewis structures.
What to do instead
Compute total valence electrons first, then distribute: bonds first, then lone pairs to satisfy octets, then check formal charge. Classify bond type using electronegativity difference: roughly less than 0.4 nonpolar covalent, 0.4 to 1.7 polar covalent, greater than 1.7 ionic. Remember that a polar covalent bond does not make the molecule polar if symmetry cancels dipoles (CO2 is nonpolar).
Predicting boiling/melting points or solubility from molecular size alone, ignoring hydrogen bonding and intermolecular force type, e.g., ranking H2O below H2S or expecting CH4 to dissolve in water.
Students conflate molecular mass with intermolecular strength and do not systematically identify the dominant force (London dispersion, dipole-dipole, hydrogen bonding) before ranking.
What to do instead
Rank in a fixed sequence: (1) identify all forces present, (2) hydrogen bonding beats dipole-dipole beats London dispersion when masses are comparable, (3) only use molar mass to break ties within the same force type. For solubility, apply 'like dissolves like' and check for H-bonding with water. Practice ranking sets like H2O, H2S, H2Se and CH4, CH3OH, CH3Cl.
Balancing an equation and then using the unbalanced mole ratios in stoichiometry, or failing to identify the limiting reactant before computing product mass.
Students rush from the given mass straight to molar mass division without confirming the equation is balanced, and they assume the reactant with the smaller mass is limiting.
What to do instead
Always write and balance the equation before any math, then convert grams to moles for every reactant, divide each by its coefficient, and pick the smallest quotient as limiting. Carry units through dimensional analysis and verify with a mass-balance check. On Regents items, also confirm whether the question asks for grams, moles, or liters at STP and use 22.4 L/mol only for gases at STP.
Confusing reaction types and net ionic equations: labeling a double-replacement precipitate reaction as single replacement, or writing spectator ions in the net ionic equation.
Students classify by surface features (two compounds on the left) rather than by what actually happens, and they do not use a solubility table to decide which product is the precipitate.
What to do instead
Classify by pattern: A + B yields AB (synthesis), AB yields A + B (decomposition), A + BC yields AC + B (single replacement), AB + CD yields AD + CB (double replacement), hydrocarbon + O2 yields CO2 + H2O (combustion). For net ionic, write the full ionic equation, cancel ions appearing unchanged on both sides, and keep only the precipitate, gas, or water.
Misreading potential energy diagrams or equilibrium shifts: calling an exothermic reaction endothermic because the diagram 'goes up', or predicting Le Chatelier shifts in the wrong direction when pressure or temperature changes.
Students read the diagram left to right visually rather than comparing reactant and product energy levels, and they apply Le Chatelier rules mechanically without tracking which side has more gas moles or whether the reaction is exothermic.
What to do instead
On PE diagrams, compare the energy of reactants vs. products: products lower means exothermic and delta H negative; activation energy is reactants to peak. For equilibrium, write the reaction with delta H on the correct side, count gas moles on each side, then apply: adding reactant shifts right, increasing pressure shifts toward fewer gas moles, increasing temperature shifts away from the heat term. Check with a quick 'which way does the system relieve the stress' sentence.
In organic and nuclear items, misapplying naming or decay rules: numbering a carbon chain from the wrong end, missing a functional group priority, or writing a beta decay product without changing the atomic number.
Students number chains left to right by habit, memorize functional group names without priority order, and treat nuclear equations as simple mass balancing without conserving charge and atomic number.
What to do instead
For IUPAC names, find the longest chain, number from the end nearest the first branch or highest-priority group (acids, aldehydes, ketones, alcohols, amines outrank halides and alkyl groups), and list substituents alphabetically. For nuclear equations, balance both mass number (top) and atomic number (bottom): alpha decay subtracts 4 and 2, beta decay adds 1 to atomic number with no mass change, positron emission subtracts 1. Verify by summing both rows on each side.