Praxis Chemistry 5245 common mistakes
Praxis Chemistry 5245 · Understand the traps and how to avoid them.
When drawing Lewis structures, stops at the first plausible arrangement without checking formal charges or whether every atom has a complete octet (or a valid expanded octet for period 3+ elements).
Candidates learn a single 'connect and fill' recipe and do not run the formal-charge/octet audit that distinguishes the best resonance structure, so they pick a structure that is chemically reasonable-looking but not the lowest-energy one.
What to do instead
After every structure, run a fixed checklist: count valence electrons, assign formal charges, check octets, and compare candidate structures by minimizing formal charge and placing negative charge on the more electronegative atom. Practice on molecules with expanded octets (e.g., SF4, PCl5) and resonance (e.g., NO3-, SO3).
Predicts molecular geometry from the number of bonded atoms instead of from the total electron-domain count including lone pairs, so H2O is called linear and NH3 is called trigonal planar.
Candidates conflate electron-pair geometry with molecular geometry and forget that lone pairs occupy domains and compress bond angles even though they are invisible in the shape name.
What to do instead
Use a two-step VSEPR routine: first count electron domains (bonds plus lone pairs) to get the electron geometry, then subtract lone pairs to name the molecular geometry and adjust angles. Memorize the domain-count-to-geometry table and drill molecules with 1-3 lone pairs.
Balances half-reactions or redox equations without first assigning oxidation numbers to identify what is oxidized and reduced, then balances atoms before charge.
Candidates treat redox balancing like ordinary equation balancing and skip the oxidation-state bookkeeping that tells them which species to split and how many electrons to add.
What to do instead
Adopt a strict order: assign oxidation numbers, identify oxidized/reduced species, split into half-reactions, balance atoms other than O and H, balance O with H2O and H with H+, then balance charge with electrons before combining. Always verify that total charge balances on both sides.
Computes enthalpy of reaction by adding or subtracting bond energies or heats of formation without matching stoichiometric coefficients and signs, producing answers off by a factor or with the wrong sign.
Candidates memorize 'products minus reactants' loosely and lose track of which tabulated quantity is being used (bond dissociation energy vs. heat of formation) and how each enters the sum.
What to do instead
Write the full Hess's-law expression explicitly with coefficients before substituting numbers, and keep a sign convention note: ΔH°rxn = ΣnΔH°f(products) − ΣmΔH°f(reactants), while bond energies use bonds broken minus bonds formed. Estimate the sign first from bond-making vs. bond-breaking and check the computed answer against it.
Treats a weak acid or weak base like a strong one, using the initial concentration directly for pH without setting up an equilibrium expression with Ka or Kb.
Candidates over-generalize the strong-acid shortcut [H+] = concentration and do not recognize when the small-dissociation approximation or an ICE table is required.
What to do instead
Before calculating pH, classify the acid/base as strong or weak from a memorized list; for weak species write an ICE table with Ka/Kb, then check whether the x-approximation is valid (x less than ~5% of initial concentration) and solve the quadratic if not. Practice buffer and salt-hydrolysis problems where this distinction changes the answer.
Predicts the direction of equilibrium shift or the effect of a change on K itself, confusing a shift in position with a change in the equilibrium constant.
Candidates memorize 'shift right/left' rules without distinguishing between disturbances that change Q relative to K (concentration, pressure) and those that change K (temperature only).
What to do instead
Frame every problem as a Q-versus-K comparison: write the reaction quotient, determine how the disturbance changes Q, and state the direction needed to restore Q = K. Explicitly note that only temperature changes K, and use ΔH sign to predict which way K moves with temperature.
Uses the wrong units or conversion path in stoichiometry and solution calculations, mixing grams, moles, molarity, and volume without tracking the units through each step.
Candidates string together familiar formulas (n = m/M, M = n/V) from memory without dimensional analysis, so a volume given in mL is plugged in as liters or a molar mass is inverted.
What to do instead
Carry units through every calculation and cancel them explicitly; convert mL to L before using molarity, and write a labeled roadmap (given → moles → target) before computing. Do a magnitude sanity check on the final answer.
Reads a graph of concentration vs. time or rate vs. concentration and misidentifies reaction order or rate law from the shape without checking the linearity test for each order.
Candidates rely on visual impression rather than the diagnostic linear plots (zero order: [A] vs t; first order: ln[A] vs t; second order: 1/[A] vs t) that actually determine order.
What to do instead
For order determination, state which plot should be linear for each candidate order and compare. For rate laws, use the method of initial rates by taking ratios of rates at two concentrations to solve for the exponent, and verify the units of k match the overall order.
Designs or critiques an experiment without identifying the independent, dependent, and controlled variables, or without a control group, so the conclusion cannot be attributed to the manipulated variable.
Candidates focus on the procedure and data table and treat experimental design as a formality, missing that the exam tests whether a stated conclusion is actually supported by a controlled comparison.
What to do instead
Before evaluating any experiment, explicitly list the independent variable, dependent variable, constants, and control. When judging a conclusion, ask whether a confounding variable was uncontrolled, and practice writing one-sentence critiques that name the specific uncontrolled variable and the fix.
Ignores or misapplies laboratory safety and waste-handling rules, such as diluting acid by adding water to concentrated acid, disposing of chemicals down the sink, or skipping PPE and fume-hood requirements.
Candidates treat safety as background knowledge and answer procedural questions on efficiency or convenience rather than on the standard safety hierarchy and waste-compatibility rules.
What to do instead
Memorize the core rules: always add acid to water (never water to acid), use a fume hood for volatile or toxic reagents, wear goggles and gloves, never return unused reagent to stock, and segregate waste by compatibility for labeled disposal. Review SDS hazard categories and the location of eyewash, shower, and fire equipment.