Chemistry — Coordination Compounds + Practical Chemistry
Error notes · Aamirah Fathima · 6 October 2026 · 80 Q · 320 marks · target 285 · danger line 260 · for AITS-04 on 11 October
156
equivalent / 180 (target 156)
1. Where the 43 marks went
On the 180 scale this paper is exactly on the Chemistry target. The eight marks between her and the 285 goal are the two unmarked questions (Q3, Q67) — both answerable, one of them already solved on the page.
2. The weakness, in one line each
- Data in the stem is being read past. Q20 states "paramagnetic with two unpaired electrons" and she chose square planar dsp², which is always diamagnetic. She had answered the diamagnetic twin (Q19) correctly moments earlier. This is the most expensive habit on the paper because it turns a question she can do into a wrong answer.
- Definitions that sound alike. Bidentate vs ambidentate (Q10), sulphate vs sulphite (Q59), Al³⁺ vs Mg²⁺ by group (Q50). In each case the distinguishing word was in the question: "two donor atoms", "does not dissolve", "NH₄Cl and NH₄OH".
- Naming prefixes. Q4 — bis/tris for ligands whose own names contain di or tri. A one-rule fix.
- Counting statements. Q17 — she rejected the false D but accepted the false C. Geometrical isomerism needs two different ligands AND an octahedral or square planar shape.
- Transfer discipline. Q67 was fully worked in the margin and the option was never marked; Q3 was skipped and never revisited. Five papers in a row have now lost marks this way.
3. What is already strong
- Assertion–Reason 6 / 6 (Q12, 22, 38, 44, 55, 74) — the error family that appeared in every chapter paper up to the Biotechnology re-test is now clean.
- Match-the-following 6 / 6 (Q9, 11, 28, 53, 62, 73), including the group reagents and the anion tests, where rows C and D used to be swapped.
- Crystal field and magnetism Q23–Q35: 13 / 13 — CFSE values, spin-only moments, high/low spin box diagrams, EAN and stability constants.
- Organic functional-group tests Q68–Q77: 10 / 10. Werner's theory and the AgCl counting block Q5–Q9: 5 / 5.
4. Question by question
Coordination Compounds — 4 wrong · 1 blank
Q3BlankTBlank — skippedWriting the formula from the name
The formula of potassium trioxalatoaluminate(III) is?
Her answerLeft blank
Correct answer(B) K₃[Al(C₂O₄)₃]
WhyThree oxalates (2− each) on Al³⁺ gives [Al(C₂O₄)₃]³⁻, so three potassium ions balance it.
Where it went wrong · fixSkipped on the first pass and never returned. Work it as a charge sum: metal charge + ligand charges = complex charge, then add counter ions.
Q4WrongNNaming ruleIUPAC naming — bis / tris prefixes
The IUPAC name of [Co(en)₂Cl₂]⁺ is?
Her answer(A) dichloridodi(ethane-1,2-diamine)cobalt(III) ion
Correct answer(C) dichloridobis(ethane-1,2-diamine)cobalt(III) ion
WhyA ligand name that already contains a numerical prefix — ethane-1,2-diamine, ethylenediamine, triphenylphosphine — takes bis, tris or tetrakis instead of di, tri, tetra, and the ligand name goes in brackets.
Where it went wrong · fixShe had the ligand order and the oxidation state right; only the multiplying prefix was wrong. Card: 2 en → bis(ethane-1,2-diamine) · 3 en → tris(ethane-1,2-diamine) · 2 NH₃ → diammine (no bracket needed).
Q10WrongNDefinitionBidentate vs ambidentate
A ligand that binds the central atom through two donor atoms is called?
Her answer(A/C) ambidentate
Correct answer(D) bidentate
WhyBidentate = both donor atoms bind at the same time (C₂O₄²⁻, en). Ambidentate = the ligand has two possible donor atoms but uses only ONE at a time (NO₂⁻ through N or O, SCN⁻ through S or N).
Where it went wrong · fixShe answered the matching question on the same idea correctly two questions later (Q11, NO₂⁻ → ambidentate). Definition card: bi = both bind · ambi = either one binds.
Q17WrongCStatement judgementStatement count — isomerism
How many are correct? A [Pt(NH₃)₂Cl₂] shows geometrical isomerism · B [Co(en)₃]³⁺ shows optical isomerism · C [Co(NH₃)₆]³⁺ shows geometrical isomerism · D tetrahedral [MA₂X₂] shows geometrical isomerism.
Her answer(C) Only three
Correct answer(B) Only two
WhyA and B are true. C is false: six identical ligands give only one arrangement, so no geometrical isomerism. D is false: a tetrahedron has no cis or trans positions — every pair of corners is adjacent.
Where it went wrong · fixShe correctly crossed D but counted C as true. Rule: geometrical isomerism needs at least TWO different kinds of ligand, and an octahedral or square planar geometry.
Q20WrongCIgnored given dataMagnetism decides the geometry
[NiCl₄]²⁻ is paramagnetic with two unpaired electrons. Its geometry and hybridisation are?
Her answer(B) square planar, dsp²
Correct answer(D) tetrahedral, sp³
WhyFor Ni²⁺ (d⁸), square planar dsp² forces all eight electrons to pair, so it is always diamagnetic. Two unpaired electrons can only come from the tetrahedral sp³ arrangement, which Cl⁻ (a weak field ligand) produces.
Where it went wrong · fixThe magnetic data in the stem is the answer key. She answered the diamagnetic case correctly in Q19 ([Ni(CN)₄]²⁻ → dsp²) and then ignored the word paramagnetic here. Read the magnetism first, then choose the geometry.
Practical Chemistry — 3 wrong · 1 blank
Q50WrongNGroup placementGroup III confirmatory test
A white gelatinous precipitate with NH₄Cl and NH₄OH, which gives a blue lake with litmus, confirms?
Her answer(A) Mg²⁺
Correct answer(B) Al³⁺
WhyThe gelatinous Al(OH)₃ adsorbs litmus to give the blue lake. Magnesium is a Group VI cation, precipitated by disodium hydrogen phosphate, not by NH₄OH.
Where it went wrong · fixThe reagent names the group: NH₄Cl + NH₄OH is Group III, which contains Al³⁺ and Fe³⁺ only. Mg²⁺ cannot appear there.
Q57WrongNColour recallHalide fumes with concentrated H₂SO₄
Violet fumes on warming a salt with concentrated sulphuric acid indicate?
Her answer(C) bromide
Correct answer(B) iodide
WhyIodide → violet fumes of I₂ · bromide → reddish-brown fumes of Br₂ · chloride → colourless, pungent HCl gas.
Where it went wrong · fixColour card for the halides, in order: colourless → reddish-brown → violet as you go down the group.
Q59WrongNPlanted wordSulphate vs sulphite
A white precipitate with barium chloride that does not dissolve in dilute hydrochloric acid confirms?
Her answer(C) sulphite
Correct answer(B) sulphate
WhyBaSO₄ is insoluble in dilute acid; BaSO₃ and BaCO₃ both dissolve in it with effervescence. The acid test is exactly what separates them.
Where it went wrong · fixThe phrase 'does not dissolve in dilute HCl' is the whole question. Underline the negative words before reading the options.
Q67BlankTBlank — not transferredStatement count — detection of elements
How many are correct? A fusion converts N, S and halogen into ionic compounds · B the sodium extract should be alkaline · C lead acetate gives a black precipitate with sulphur · D phosphorus is detected with sodium nitroprusside.
Her answerLeft blank
Correct answer(C) Only three
WhyA, B and C are true; D is false — phosphorus is detected with ammonium molybdate, giving a yellow precipitate. Sodium nitroprusside is the sulphur test.
Where it went wrong · fixShe solved it and did not transfer the answer. The statements are ticked A, B, C with D crossed out on the paper, which gives 'only three' — but no option was marked. Four marks for work already done.
5. Fix list before 11 October
- Underline the data in the stem before reading the options: paramagnetic / diamagnetic, number of unpaired electrons, "does not dissolve", "only one", "NOT". Q20, Q59 and Q17 all turn on a single such word.
- Mark the option the moment a question is finished. No question is left for later — Q67 was solved and scored zero.
- Five cards to carry: bis / tris for ligands containing di or tri · bi = both bind, ambi = either binds · paramagnetic Ni²⁺ → tetrahedral sp³, diamagnetic → square planar dsp² · violet fumes = iodide, reddish-brown = bromide, colourless = chloride · BaSO₄ insoluble in dilute acid, BaSO₃ and BaCO₃ dissolve.
- Geometrical isomerism test: two different kinds of ligand AND octahedral or square planar. Six identical ligands or a tetrahedron → none.
- Re-solve Q3, 4, 10, 17, 20, 50, 57, 59, 67 cold in the evening block, then one 20-question set on naming, isomer counting and the halide/sulphate tests.