Traps
How MHT-CET Chemistry loses you marks even when you know the chemistry
Only 3% of this paper is HARD, so marks are not lost to difficulty. They are lost to misreading — a look-alike reagent, a unit, an exception — and to time: the penalty for a wrong answer is 0, and Chemistry shares a 90-minute paper with Physics, at about 0.7 minutes a Chemistry question.
- trap shapes
- 13
- strands affected
- 3
- playbooks hit by the widest trap
- 4
- paper-wide, not chapter-specific
- 2
How to use this page
Read it once cover-to-cover, then re-read the section matching your next practice block — a trap is far easier to spot when you have just been primed on its mechanism. One thing to know before you start: a trap is filed under the strand whose MARKS it costs you, not the strand the question sits in. Starting the paper with the calculations is filed under Calculate, though what it costs you is the Recall questions you reach too late.
Calculate traps — arithmetic and units
Two paper-wide habits and three arithmetic slips. The chemistry here is not hard; the marks go to a unit, a factor or a sign.
Leaving a question blank — there is no negative marking
Paper-wide — not tied to any one chapter
The mechanic
A blank and a wrong answer both score zero on MHT-CET. The habit carried in from exams that deduct marks — leave it if unsure — turns a free attempt into a certain zero.
The fix
Keep the last three minutes of Paper II for filling every blank. Eliminate what you can, then choose.
Starting with the calculations and running out of time for the recall
Paper-wide — not tied to any one chapter
The mechanic
About 25 Chemistry questions a paper are recall or reactions — answered in seconds — and about 21 need arithmetic. Working the paper in order spends the first minutes on calculations and reaches the easy names and structures with the clock already short.
The fix
First sweep: every recall and reaction question. Second sweep: the calculations. One mark each either way; the fast marks first.
A unit left unconverted — pm, dm³ bar, kJ against J
Affects: Solid State, Chemical Thermodynamics, Electrochemistry
The mechanic
Solid State edges come in pm and density wants cm; thermodynamic work comes in dm³ bar and the answer in J; ΔG° = −nFE° comes out in J while the options are in kJ. The unconverted answer is always printed.
The fix
Write the unit next to every number. 1 pm = 10⁻¹⁰ cm; 1 dm³ bar = 100 J; divide by 1000 for kJ.
Forgetting the van't Hoff factor for an electrolyte
Affects: Solutions and Colligative Properties
The mechanic
Every colligative property scales with the number of particles. NaCl gives two, CaCl₂ three, AlCl₃ four. Leaving i = 1 gives the answer for glucose, and it is an option.
The fix
Before any colligative calculation, write i: 1 for a non-electrolyte, the ion count for a strong electrolyte.
The wrong n in the Nernst equation
Affects: Electrochemistry
The mechanic
E = E° − (0.0592/n) log Q. For Zn²⁺ or Cu²⁺, n = 2: a tenfold change moves the potential by 0.0296 V, not 0.0592 V. Ten of the Electrochemistry HARD questions are Nernst arithmetic.
The fix
Read n from the balanced half-reaction — the number of electrons — before substituting.
Reactions traps — the reagent that looks like another
Each of these is a pair of look-alikes — a reagent, a condition, a carbon count — where the wrong one's product is printed beside the right one's.
Two reagents that look alike and do different things
Affects: Aldehydes, Ketones and Carboxylic Acids, Halogen Derivatives of Alkanes, Alcohols, Phenols and Ethers
The mechanic
Aqueous KOH substitutes and alcoholic KOH eliminates. Clemmensen is acidic and Wolff–Kishner basic. Finkelstein makes iodides and Swarts fluorides. The product of the look-alike is always among the options.
The fix
Learn reactions as reagent → product pairs, and note the one word that separates look-alikes: aqueous or alcoholic, acid or base, iodide or fluoride.
Losing or keeping a carbon
Affects: Amines, Alkanes, Aromatic Compounds
The mechanic
Hofmann bromamide and decarboxylation remove one carbon; Wurtz doubles the chain; side-chain oxidation cuts any alkyl group down to –COOH. A product with the wrong carbon count is the planted option.
The fix
Count carbons before and after. Ask: does this reaction add, remove or keep them?
Aldol against Cannizzaro — the α-hydrogen decides
Affects: Aldehydes, Ketones and Carboxylic Acids
The mechanic
Aldol condensation needs a hydrogen on the carbon next to C=O; Cannizzaro works only without one. HCHO and benzaldehyde have none; ethanal has three.
The fix
Look at the α-carbon first. Hydrogen present: aldol. None: Cannizzaro.
The gas-phase basicity order in a water question
Affects: Amines
The mechanic
In the gas phase, tertiary amines are the strongest bases. In water, solvation reverses part of that: for methylamines, 2° > 1° > 3°. The gas-phase order is printed as an option.
The fix
Unless the question says gas phase, use the aqueous order.
Recall traps — the exception, not the trend
Recall questions are built on exceptions and fine print. Learning the trend and missing its exception is the most common way a first-sweep mark is lost.
Applying a trend where the paper asks for its exception
Affects: Transition and Inner Transition Elements, Elements of Group 16, 17 and 18, Elements of Group 1 and 2, Redox Reactions
The mechanic
Recall questions are built on the exceptions: Cr and Cu configurations, water's boiling point, lithium and beryllium's anomalies, peroxide oxygen at −1. The trend answer is the planted one.
The fix
Keep a one-page list of exceptions and read it before every mock. The trend is not what is being tested.
Counting ligands where the question counts donor atoms
Affects: Coordination Compounds
The mechanic
[Co(en)₃]³⁺ has three ligands and coordination number 6, because each en binds twice. Denticity is 33 questions of Coordination Compounds.
The fix
Coordination number = number of donor atoms bonded to the metal, not number of ligands.
Calling sucrose a reducing sugar
Affects: Biomolecules
The mechanic
Sucrose's glycosidic link joins both anomeric carbons, leaving no free hemiacetal, so it does not reduce Fehling's or Tollens' reagent. Maltose and lactose do. 'Which is non-reducing?' is a standing question.
The fix
Learn the linkages as a table: sucrose α1–β2 (non-reducing), maltose α1–4, lactose β1–4.
Coagulating a sol with an ion of the same charge
Affects: Surface Chemistry
The mechanic
The Hardy–Schulze rule is about the ion OPPOSITE in charge to the sol. A large, highly charged anion does nothing to a negative sol, and it is offered as the answer.
The fix
Find the sol's charge first, then rank only the opposite-charged ions by their charge.
The two habits that cover most of this page
Most of the 13 shapes above reduce to one of two disciplines, and neither of them is extra chemistry.
Before you start writing
Read the one word that decides it
Aqueous or alcoholic? Acid or base? Electrolyte or not? pm or cm? The trend, or its exception? Most wrong answers here come from one word read past. Fifteen seconds of reading is cheaper than a wrong mark, and at about 0.7 minutes a question there is no time to come back.
Before you hand the paper in
Nothing goes back unanswered
Mark every uncertain question as you pass it, and keep the last three minutes of Paper II free to fill whatever is still empty. A blank and a wrong answer score the same on this exam, so an unfilled bubble is the only error here that costs you marks with certainty rather than probability.