Assam ASSEB HS Chemistry common mistakes
Assam ASSEB HS Chemistry · Understand the traps and how to avoid them.
In unit-cell density and packing calculations, substituting an edge length given in pm directly into d = ZM/(a^3 N_A) and reporting g cm^-3, or using r = a/2 for an fcc cell.
The density formula is memorised without unit conversion, and the atom-count/radius relation for each lattice is mixed up.
What to do instead
Convert pm to cm before cubing (1 pm = 10^-10 cm), write Z explicitly, and use the correct relation: sc r = a/2; bcc 4r = √3a; fcc 4r = √2a. Check that the final unit is g cm^-3.
Calculating boiling-point elevation or freezing-point depression for NaCl, CaCl2 or glucose with the same formula and no van't Hoff factor, or using molarity instead of molality.
Candidates treat every solute as a non-electrolyte and use the more familiar molarity concentration term.
What to do instead
Classify the solute, assign i (NaCl = 2, CaCl2 = 3, glucose = 1; weak electrolytes use degree of dissociation), convert to molality, then apply ΔT = iK m. Check the sign for freezing-point depression.
In Nernst-equation numericals, inserting concentrations into E = E° − (0.059/n) log Q with the wrong n, reversing anode and cathode, or using ln instead of log10.
Half-reactions are not balanced first, so Q and n do not match the overall cell reaction.
What to do instead
Write and balance both half-reactions, determine n from electrons transferred, construct Q for the overall reaction, use 0.059 V only with log10 at 298 K, and confirm E > 0 for the spontaneous direction.
Deciding reaction order from stoichiometric coefficients and then using the first-order half-life t1/2 = 0.693/k even when the units of k or the data show zero or second order.
Order and molecularity are conflated, and the integrated rate law is selected from the chemical equation rather than from experimental data.
What to do instead
Determine order from experimental rate data or from the units of k (M s^-1 zero, s^-1 first, M^-1 s^-1 second); use the matching integrated law and plot test before calculating half-life.
Writing group trends as universal rules: stating all Group 15 elements show +5 equally, all halogens form stable +7 oxoacids, or HF is a strong acid like HCl.
Anomalies arising from small size, poor shielding, d-orbital availability and the inert-pair effect are memorised as isolated facts rather than explained.
What to do instead
For each group, learn the named anomalies (N vs P, O vs S, F vs Cl, inert pair in Tl/Pb/Bi) and justify them using size, electronegativity and d-orbital availability; write supporting equations such as HF dissociation and H2O behaviour.
Predicting transition-metal ion electronic configurations by removing 4s electrons last, or explaining lanthanoid contraction as simple outside attraction without mentioning poor 4f shielding.
Neutral-atom exceptions such as Cr and Cu are memorised, but ion formation and f-orbital shielding are not practised.
What to do instead
Write the neutral configuration, then for cations remove 4s electrons before 3d; use examples such as Fe2+, Fe3+ and Ce4+. Explain lanthanoid contraction by poor shielding of 4f electrons, increasing effective nuclear charge.
Naming coordination compounds without assigning the metal oxidation state from charge balance, or counting cis/trans isomers for a tetrahedral complex.
Nomenclature rules and geometry/isomerism links are learned separately, so charge and geometry are not checked before naming or isomer counting.
What to do instead
Use charge balance to assign the metal oxidation state, name ligands alphabetically with appropriate prefixes, and identify geometry first: tetrahedral complexes have no cis/trans isomers, while square planar and octahedral complexes may.
Choosing SN1 versus SN2 solely from 'tertiary' or 'primary' and ignoring solvent, nucleophile strength and elimination competition; for example, treating a tertiary alkyl halide with a strong bulky base as SN1.
The mechanism is reduced to one variable, and E1/E2 or solvolysis conditions are not integrated into the decision.
What to do instead
Use a decision matrix: substrate, nucleophile/base strength, solvent proticity and temperature. Primary + strong nucleophile in polar aprotic solvent favours SN2; tertiary in polar protic solvent favours SN1; strong bulky base with heat favours elimination; check for rearrangements.
Using Lucas, iodoform, Fehling's or Tollens' tests without specifying reagent conditions and expected observation; for example, expecting Fehling's to oxidise benzaldehyde or using Tollens' to distinguish all ketones from all aldehydes without observation details.
Tests are memorised as names rather than as reagent-specific reactivity, so the correct positive/negative result is not linked to structure.
What to do instead
Write the reagent and observation: Lucas uses anhydrous ZnCl2/conc HCl and gives turbidity 3° > 2° > 1°; iodoform uses I2/NaOH and requires CH3CO– or CH3CH(OH)–; Fehling's oxidises aliphatic aldehydes but not aromatic aldehydes; Tollens' gives a silver mirror with aldehydes, not ketones.
In volumetric analysis, calculating molarity from a titre without balancing the titration equation or applying the stoichiometric ratio; in salt analysis, adding group reagents in the wrong order or ignoring flame-test confirmation.
The practical procedure is memorised as a sequence of steps without mole ratios or the logic of group separation and confirmatory tests.
What to do instead
Write the balanced titration equation and use mole ratios; apply M1V1/n1 = M2V2/n2 only after balancing. In salt analysis, follow group reagent order (dilute HCl, H2S in acidic medium, NH4OH, etc.) and confirm cations with flame or specific confirmatory tests, recording each observation.