Armenia Chemistry Unified Exam common mistakes
Armenia Chemistry Unified Exam · Understand the traps and how to avoid them.
When writing electron configurations of transition-metal atoms and ions, filling 4s before 3d for ions (e.g., writing Fe²⁺ as [Ar]4s²3d⁴) instead of removing 4s electrons first to get [Ar]3d⁶.
Students memorize the aufbau order as a fixed filling sequence and apply it unchanged to ions, not realizing that 4s is higher in energy than 3d in the actual ion and that electrons are removed from the highest-n subshell first.
What to do instead
Drill a fixed rule: build the neutral atom by aufbau, then for cations remove electrons from the outermost n (4s before 3d). Practice Fe, Fe²⁺, Fe³⁺, Cu, Cu²⁺, Cr, Cr³⁺ until the pattern is automatic.
Predicting molecular geometry from the number of bonded atoms only, calling H₂O linear or NH₃ trigonal planar, without counting lone pairs on the central atom.
Students conflate electron-domain geometry with molecular geometry and forget that lone pairs occupy domains and compress bond angles, so they skip the lone-pair count.
What to do instead
Use a fixed VSEPR worksheet: count total electron domains, subtract bonding domains to get lone pairs, assign AXₘEₙ label, then state shape and approximate angle. Apply to H₂O (bent ~104.5°), NH₃ (trigonal pyramidal ~107°), SO₂, SF₄.
Treating all "increase in atomic radius / metallic character / basicity of oxides" trend questions as monotonic across a period and down a group without checking the specific comparison, e.g., claiming radius always increases left-to-right.
The trends are taught as slogans ("radius increases down and to the left") and students apply them in the wrong direction or forget that radius decreases across a period because of increasing effective nuclear charge.
What to do instead
Anchor each trend to its cause: Z_eff rises across a period → radius shrinks, ionization energy rises, metallic character falls; n rises down a group → radius grows, ionization energy falls. Then answer by locating both elements on a blank periodic table sketch before choosing.
Naming or writing formulas of coordination compounds without balancing the charge of the complex ion, e.g., writing [Cu(NH₃)₄]Cl instead of [Cu(NH₃)₄]Cl₂, or giving the wrong oxidation state of the central metal.
Students focus on the ligand count and forget that the counter-ions must make the whole compound neutral, and they do not systematically compute the metal's oxidation state from ligand charges.
What to do instead
Always compute oxidation state first: sum of ligand charges plus x must equal the complex charge. For [Cu(NH₃)₄]²⁺, x + 0 = +2 so Cu is +2 and two Cl⁻ are needed. Practice with [Fe(CN)₆]³⁻, [Ag(NH₃)₂]⁺, K₄[Fe(CN)₆].
Solving limiting-reagent problems by comparing masses of reactants directly instead of converting to moles and comparing mole ratios to the balanced equation.
Students shortcut to mass comparison because it feels intuitive, but the stoichiometric coefficients make the mole ratio, not the mass, the deciding factor.
What to do instead
Enforce a 3-step template: (1) convert every reactant to moles, (2) divide each by its coefficient, (3) the smallest quotient is limiting. Only then compute product mass from the limiting reagent. Practice with a reaction where the larger-mass reactant is limiting.
Applying Le Chatelier's principle to pressure changes by counting total moles on each side without distinguishing gases from solids/liquids, or claiming a catalyst shifts the equilibrium position.
Students treat every species as a gas and assume any disturbance must shift the equilibrium; they also confuse the effect of a catalyst on rate with an effect on position.
What to do instead
Before answering, list only gaseous species and their coefficients. State the rule: pressure increase favors the side with fewer gas moles; equal gas moles means no shift; a catalyst changes only the time to reach equilibrium.
In acid–base calculations, mixing strong and weak acid formulas, or forgetting that a weak acid does not dissociate completely when computing pH, e.g., using [H⁺] = c for CH₃COOH.
Students apply the strong-acid formula (full dissociation) to every acid and do not use Ka or the square-root approximation for weak acids.
What to do instead
Classify first: strong acid → [H⁺] = c; weak acid → [H⁺] = √(Ka·c) (checking the approximation), then pH = −log[H⁺]. Practice side by side with HCl and CH₃COOH at the same concentration to see the pH difference.
Balancing redox equations in acidic or basic medium without adding H₂O and H⁺/OH⁻ to balance O and H, producing equations that balance atoms of the main elements but not charge.
Students balance only the elements they see and forget that the half-reaction method requires adding water and protons to close the oxygen and hydrogen balance, then checking total charge.
What to do instead
Use the half-reaction checklist: balance main atoms, balance O with H₂O, balance H with H⁺, balance charge with electrons, then combine and cancel. In base, add OH⁻ to both sides to neutralize H⁺. Always verify total charge on each side.
Drawing structural formulas of organic compounds with wrong carbon valence (e.g., a carbon with three bonds and no charge) or misapplying Markovnikov's rule when predicting addition products.
Students focus on the functional group and neglect the tetravalency of carbon, and they memorize Markovnikov's rule as a phrase without applying it to the specific alkene.
What to do instead
After drawing any structure, count bonds on every carbon and add H until four. For addition, identify the more substituted carbon of the double bond and place H on the carbon with more hydrogens (or use carbocation stability). Practice with propene + HBr and 2-methylbut-2-ene + HCl.
Reporting a calculated result with more significant figures than the data allow, or omitting units and the correct unit conversion (e.g., using mL directly in a molarity calculation instead of converting to L).
Students treat the calculator output as the answer and do not track units through the calculation, so volume in mL is substituted into c = n/V without conversion.
What to do instead
Write the unit on every quantity, convert to SI (mL → L, g → mol via molar mass) before substituting, and round the final answer to the least number of significant figures in the given data. Check that units cancel to give the expected unit.