Solutions JEE Main PYQ — Colligative Properties Are 55 Percent and the van't Hoff Factor Decides Them (2015-2026)
Solutions JEE Main PYQ (2015-2026), 240+ questions decoded. Colligative properties are 55%, the van't Hoff reverse-engineering archetype, the association trap, and 15 PYQs.
One Number Runs This Whole Chapter: the van't Hoff Factor.
If you master one thing in Solutions, make it the van't Hoff factor (i). It's the multiplier that sits inside every colligative-property formula, and getting it wrong is the single biggest source of lost marks in the chapter. The examiners know this, which is why the guaranteed archetype is i-factor reverse engineering: they give you the observed property (ΔTf, ΔTb, or π) and make you work backward to the degree of dissociation, the molar mass, or the coordination sphere. Nearly every session has one.
Here's why this chapter matters more than it used to. After the rationalisation deleted several Physical Chemistry chapters, Solutions climbed to share the top of the section with Thermodynamics and Electrochemistry — 1-2 questions per shift, 4-8 marks. And it's calculation-heavy in exactly the way the new exam punishes: colligative properties alone are 54.5% of the chapter, almost all of it numerical. Since all 5 NVQs became compulsory with −1 in 2025, you can no longer skip these dense calculations — Solutions is now a primary NVQ battleground.
The good news: it's formula-driven and predictable. A finite set of formulas, a handful of recurring archetypes, and a fixed list of traps. Master those and 4-8 marks become reliable. We analysed 240+ Solutions questions across every JEE Main shift from 2015 to 2026. This is Logic Bloom's eighth JEE Main PYQ analysis, after Thermodynamics, Coordination Compounds, and six others.
| 🎯 We analyzed 240+ JEE Main Solutions questions. The app has them all — ready to play and practice. | |
|---|---|
| This chapter is won by calculation fluency — the same archetypes, solved until the workflow is automatic. Logic Bloom's Playground turns Solutions into interactive practice: dissolve a solute and watch the freezing point drop as i changes, mix volatile liquids and read the vapour-phase composition, reverse-engineer a coordination sphere from a ΔTf value. Then drill every PYQ including the compulsory NVQ type. When an association-vs-dissociation slip or a kbar unit trap catches you, TarQ teaches the fix, and your Mistake Book logs it. | Get the app → Free to start. |
Sub-Topic Frequency: Colligative Properties Dominate
| Sub-topic | Share | Priority |
|---|---|---|
| Colligative properties & van't Hoff factor | 54.5% | Very High |
| Raoult's law & vapour pressure | 26.8% | Very High |
| Solubility & concentration terms | 13.4% | Moderate |
| Henry's law | 5.3% | Moderate |
Two clusters are 81% of the chapter. Within colligative properties, depression of freezing point (ΔTf) is the single most-tested property — because it couples so cleanly with weak-acid Ka (ionic equilibrium) and with coordination-complex ionisation. Osmotic pressure is second, favoured for isotonic comparisons and polymer molar-mass problems. Pure concentration interconversions have nearly vanished as standalone questions — they're now just step one of a longer NVQ.
The Format That Raised the Stakes: 60% NVQ and Climbing
| Era | MCQ : NVQ | What Changed |
|---|---|---|
| Early NVQ (pre-2024) | 70 : 30 | NVQs optional, could be skipped |
| 2024-2026 | ~40 : 60 | All 5 NVQs compulsory, −1 penalty (2025) |
Solutions is now weaponised. The NTA raises difficulty not through obscure concepts but through cognitive load and calculation intensity — awkward numbers (Kf = 1.86, molar masses like 174 or 342) that demand rigorous calculator-free arithmetic, with tight NVQ tolerances that punish rounding. Since you can no longer skip Section B, this chapter's dense calculations are unavoidable. Accuracy under time pressure is the whole game.
| 🎯 Benzoic acid in benzene gives a van't Hoff factor of 0.5, not 2. Students who use the dissociation formula walk straight into the distractor. | |
|---|---|
| The association trap. Almost every solute in this chapter dissociates — NaCl gives i=2, K₄[Fe(CN)₆] gives i=5 — so students reach for i = 1 + (n−1)α on autopilot. But benzoic acid and acetic acid associate into dimers in benzene, and that needs the OTHER formula: i = 1 + (1/n − 1)α. For 100% dimerisation, i = 0.5, not 2. The colligative property HALVES instead of doubling. The examiners always include the dissociation-based answer as a distractor, so using the wrong formula lands you exactly on a wrong option. The tell: a carboxylic acid in a non-polar solvent like benzene means association, not dissociation. Logic Bloom's Playground shows the dimers forming so you see why i drops below 1 — with TarQ explaining which formula applies. Then drill every PYQ and let your Mistake Book catch the i-factor slips. | See the dimers form → Free to start. |
The Four Colligative Properties — Your Core Reference
Every one carries the van't Hoff factor i. Memorise the formula and the molar-mass expression for each:
| 🎯 The Four Colligative Properties | |||
|---|---|---|---|
| Property | Formula | Constant | Molar Mass (M₂) |
| RLVP | (p°−p)/p° = i·x₂ | — | M₂ = (W₂·M₁·p°)/(W₁·(p°−p)) |
| Boiling-point elevation | ΔTb = i·Kb·m | Kb (ebullioscopic) | M₂ = (1000·Kb·W₂)/(ΔTb·W₁) |
| Freezing-point depression | ΔTf = i·Kf·m | Kf (cryoscopic) | M₂ = (1000·Kf·W₂)/(ΔTf·W₁) |
| Osmotic pressure | π = i·C·R·T | R (gas constant) | M₂ = (W₂·R·T)/(π·V) |
The Kb vs Kf trap: for water, Kf = 1.86 is much larger than Kb = 0.52. Any statement claiming Kb > Kf is false. And don't confuse the final boiling temperature (100.52°C) with the magnitude of the change (0.52).
The van't Hoff Factor Reference — Know These i Values
| 🎯 van't Hoff Factor (i) for Common Solutes | ||
|---|---|---|
| Solute type | Example | i (α = 1) |
| Non-electrolyte | Glucose, urea, sucrose | 1 |
| Binary strong electrolyte | NaCl, KCl | 2 |
| Ternary strong electrolyte | BaCl₂, K₂SO₄, CaCl₂ | 3 |
| Complex ion | K₄[Fe(CN)₆] | 5 |
| Associating solute | Benzoic acid in benzene (dimer) | 0.5 |
Dissociation: i = 1 + (n−1)α. Association: i = 1 + (1/n − 1)α. The reverse-engineering archetype gives you the observed property, you find i = observed/calculated, then solve for α or the molar mass. For the CrCl₃·xNH₃ coordination question, i tells you how many ions the complex releases — which fixes the coordination sphere (this cross-links directly to our JEE Coordination Compounds analysis).
Raoult's Law and Deviations
| 🎯 Ideal vs Non-Ideal Solutions | |||
|---|---|---|---|
| Type | A-B interaction | Azeotrope | Examples |
| Ideal | A-B = A-A = B-B (ΔH=0) | None | Benzene + Toluene |
| Positive deviation | A-B weaker (ΔH > 0) | Minimum boiling | Ethanol + Acetone, CS₂ + Acetone |
| Negative deviation | A-B stronger (ΔH < 0) | Maximum boiling | Chloroform + Acetone, Phenol + Aniline |
The vapour-phase trap: when asked for a component's mole fraction in the VAPOUR phase, use y_A = p_A/p_total — NOT the liquid mole fraction x_A. Examiners bank on students calculating x_A quickly and picking it. Total pressure: p_total = p°_A·x_A + p°_B·x_B.
The Five Traps That Cost Marks
| 📌 Where Candidates Lose Marks (−1 Each Under NVQ) | |
|---|---|
| 1. The association trap | Using i = 1 + (n−1)α for a dimerising solute. Benzoic/acetic acid in benzene associate → i = 1 + (1/n−1)α → i = 0.5 at full dimerisation. |
| 2. The kbar unit trap | Henry's law with KH in kbar — forgetting to convert to bar makes the answer off by a factor of 1000. |
| 3. Celsius in osmotic pressure | π = iCRT needs T in KELVIN. Leaving it in °C is the most common −1 in the chapter. |
| 4. Vapour vs liquid mole fraction | Answering with x_A (liquid) when the question asks y_A (vapour). Use y_A = p_A/p_total. |
| 5. The density shortcut | Assuming density = 1 g/mL to convert molarity↔molality when a density IS given (e.g. 1.02). Tight NVQ tolerance rejects the shortcut answer. |
Cross-Chapter Integration (the Real Difficulty)
| Combination | What It Tests |
|---|---|
| Solutions + Coordination Compounds | Reverse-engineer the coordination sphere from ΔTf (the CrCl₃·xNH₃ archetype). The dominant rising combination. |
| Solutions + Ionic Equilibrium | Weak acid + freezing point: find α from Ka via Ostwald's law, then i = 1 + α, then ΔTf. |
| Solutions + Thermodynamics | Ideal-solution enthalpy and entropy of mixing (ΔHmix = 0). |
| Solutions + Mole Concept | Concentration math as the hidden step-one of a multi-step colligative NVQ. |
JEE Main 2027 / 2028 Predictions
Predictions exclude deleted content.
Top 5 Sub-Topics Most Likely to Appear
| # | Predicted Topic | Why |
|---|---|---|
| 1 | Coordination + colligative (NVQ) | Isomers differentiated by π or ΔTf — the fastest-rising combination. |
| 2 | Vapour-phase composition (y_A) | Raoult + Dalton, now under non-ideal conditions. |
| 3 | Successive colligative calculations | π data → concentration → ΔTb, chained in one NVQ. |
| 4 | Henry's law + Thermodynamics | Gas solubility at two temperatures → enthalpy of dissolution. |
| 5 | Isotonicity with weak electrolytes | Equate biological fluid osmotic pressure to a weak acid → find Ka. |
3 Dormant Concepts Due for Return
| Concept | Likely Format |
|---|---|
| Ostwald-Walker method | RLVP via air bubbled through solution and solvent — absent recently, ripe for a hard NVQ. |
| Fractional distillation (T-x-y) | Reading liquid/vapour composition from an azeotrope phase diagram. |
| Kb from ΔHvap | Deriving the ebullioscopic constant thermodynamically. |
Solutions JEE Main PYQs — 15 Questions You Must Attempt
These 15 span 2015-2026 and reflect JEE's exact difficulty and NVQ style. Each has a worked one-line solution and the trap explained.
| 📌 15 Must-Attempt JEE Main Solutions PYQs | |
|---|---|
| 1. Osmotic Pressure NVQ (2026 Apr) | π of 50 mg K₂SO₄ (M=174) in 2 L water at 27°C, complete dissociation (R=0.083)? Answer: 0.0107 bar. Solution: i=3; π = 3×(0.05/174/2)×0.083×300. Trap: Forgetting i=3, or leaving mass in mg. |
| 2. Coordination i-Factor (2025 Jan) | CrCl₃·xNH₃, 0.1 m, ΔTf=0.558 K, 100% ionisation. Formula? (Kf=1.86) Answer: [Cr(NH₃)₅Cl]Cl₂. Solution: i = 0.558/(1.86×0.1) = 3 → 1 complex ion + 2 Cl⁻. Trap: Guessing the sphere from ligand count, not from i. |
| 3. Hypotonic Comparison (2026 Apr) | 0.1 M K₄[Fe(CN)₆] vs 0.1 M FeCl₃ across SPM — which is hypotonic? Answer: FeCl₃. Solution: i·C = 5×0.1 vs 4×0.1 → FeCl₃ (0.4) is lower. Trap: Letting the heavier metal decide instead of counting ions. |
| 4. Henry's Law Units (2024 Apr) | O₂ in water at 303 K, KH=46.82 kbar, p=0.920 bar. Millimoles in 1 L? Answer: ~1.09 mmol. Solution: x = p/KH with KH = 46.82×10³ bar. Trap: Not converting kbar → bar, off by 1000×. |
| 5. Kb vs Kf (2026 Apr) | Which statement is INCORRECT? Answer: "Kb for water > Kf." Solution: Kb=0.52 < Kf=1.86. Trap: Confusing the final boiling temperature with the constant's magnitude. |
| 6. Vapour-Phase Fraction (2025 Apr) | Benzene 80 Torr, methylbenzene 24 Torr, equimolar. Vapour mole fraction of methylbenzene? Answer: 0.23. Solution: y = (0.5×24)/(0.5×80+0.5×24) = 12/52. Trap: Computing benzene's fraction (0.77) instead. |
| 7. Weak Acid + FP (2026 Apr) | 19.5 g fluoroacetic acid (M=78) in 500 g water, ΔTf=1°C. Find α. (Kf=1.86) Answer: ~7.5%. Solution: m=0.5; i = 1/(1.86×0.5) = 1.075; α = i−1. Trap: Forgetting to subtract 1 from i to get α. |
| 8. Hemoglobin Molar Mass (2026 Apr) | 20 g hemoglobin in 1 L, column 80 mm higher. M in kg/mol? (ρ=1000, g=10, R=8.3) Answer: ~62 kg/mol. Solution: π = ρgh = 800 Pa; set = CRT, solve M in SI. Trap: Using atm/bar R with the ρgh pressure. |
| 9. Mass % → Mole Fraction (2026 Apr) | 30% w/w methanol in CCl₄. Mole fraction of CCl₄? Answer: 0.33. Solution: n(MeOH)=30/32, n(CCl₄)=70/154; x = 0.454/(0.454+0.937). Trap: Computing methanol's fraction instead of the solvent's. |
| 10. Isotonic Glucose (2025 Jan) | 0.9% NaCl isotonic with glucose. Glucose % (w/v)? Answer: 5.4%. Solution: 2×(0.9/58.5) = 1×(W/180) → W≈5.4. Trap: Forgetting i=2 for NaCl → wrong answer of 2.7%. |
| 11. Successive Raoult (2026 Jan) | 3 mol A + 1 mol B → 500 mmHg; add 1 mol A → 520. Find p°_B. Answer: 400. Solution: Two equations in p°_A, p°_B; solve simultaneously. Trap: Total moles become 5 after addition, not 4. |
| 12. Henry Log Graph (2026 Jan) | Plot of log p vs log x for CO₂ in water? Answer: Straight line, slope +1, intercept log KH. Solution: log p = log x + log KH. Trap: Mistaking the slope for KH — the slope is 1. |
| 13. Two Compounds ΔTb (2026 Jan) | PQ and PQ₂, given ΔTb values and Kb=5. Molar masses of P and Q? Answer: P=25, Q=60. Solution: Find M(PQ) and M(PQ₂), solve P+Q and P+2Q. Trap: Swapping P and Q in the simultaneous solve. |
| 14. RLVP from ΔTb (2026 Jan) | 1.5 g X in 150 g Y (M=300), ΔTb=0.5 K, Kb=5. RLVP? Answer: 0.03. Solution: m = ΔTb/Kb = 0.1; RLVP = m·M_solvent/1000 = 0.03. Trap: Computing molar mass of X first — unnecessary and slow. |
| 15. Molarity Density Trap (2024) | 13% w/w H₂SO₄, density 1.02 g/mL. Molarity? Answer: 1.35 M. Solution: M = (13×1.02×10)/98. Trap: Assuming density=1 → wrong answer outside NVQ tolerance. |
| 🎯 These are 15 of the 200+ JEE Main Solutions PYQs in the app. Drill all of them. | |
|---|---|
| Every question above — including the compulsory NVQ type — is inside Logic Bloom, mapped across all shifts. Reverse-engineer i-factors, calculate every colligative property, read vapour-phase composition. When a trap catches you, TarQ teaches the reasoning — not just the answer. Your Mistake Book tracks exactly which trap cost you — the association slip, the kbar unit, the Celsius-in-osmotic-pressure error. Then take it into Battleground — 1v1 duels under real exam pressure. Get Logic Bloom — Free to start → |
How to Prepare Based on the Data
| 📌 Data-Driven Strategy for JEE Main Solutions | |
|---|---|
| Master the van't Hoff factor first | It sits in every colligative formula and drives the guaranteed reverse-engineering archetype. Know both formulas — dissociation AND association — cold. |
| Learn the four-property table as one unit | Formula, constant, and molar-mass expression for each. Colligative properties are 55% of the chapter; this table is most of your marks. |
| Drill units before formulas | Kelvin in osmotic pressure, kbar → bar in Henry's law, density when given. Most wrong NVQ answers are right physics with wrong units. |
| Spot association vs dissociation instantly | A carboxylic acid in benzene associates (i < 1). Everything else in the chapter dissociates. Getting this wrong lands you on a designed distractor. |
| Practise the coordination cross-link | The CrCl₃·xNH₃ archetype (ΔTf → coordination sphere) is the fastest-rising combination. It's Solutions and Coordination in one question. |
| Reverse-engineer, don't over-calculate | Logic Bloom's Playground turns Solutions into interactive practice — dissolve solutes, mix liquids, work backward from observed properties — with TarQ teaching the reasoning. Drill every PYQ including NVQs, with your Mistake Book catching the unit and i-factor errors. Then test under pressure in Battleground. Free to start. |
Building your JEE Main Physical Chemistry base? This shares the top of the section.
| 🎯 1-2 questions per shift. Top-tier Physical Chemistry. Formula-driven and NVQ-heavy. The patterns are here. The practice is in the app. | |
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| 🎮 Playground Understand through practice — with TarQ |
Every Solutions concept as interactive practice — dissolve a solute and watch the freezing point drop, mix volatile liquids and read the vapour composition, reverse-engineer a coordination sphere from ΔTf. Drill every PYQ across all shifts, including the NVQ type. When you're stuck, TarQ teaches the reasoning. Mistake Book catches the unit and i-factor slips before the exam does. Get the app → |
| ⚔️ Battleground Score through practice — 1v1 duels |
NVQ accuracy under time pressure is what this chapter rewards. Battleground trains exactly that — timed 1v1 duels across Physics, Chemistry, Biology, ELO climbing through 6 tiers. Get the app → |
| Understand through games. Score through practice. Get Logic Bloom — Free to start → |
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FAQs — Solutions JEE Main PYQ
Q1: How many questions come from Solutions in JEE Main?
Solutions delivers 1-2 questions per shift, roughly 4-5% of the Chemistry section. After the rationalisation deleted Solid State, States of Matter, and Surface Chemistry, it rose to share the top of Physical Chemistry weightage with Thermodynamics and Electrochemistry, reliably generating 4-8 marks per shift.
Q2: What is the most important concept in JEE Main Solutions?
The van't Hoff factor (i). It appears inside every colligative-property formula, and the guaranteed archetype is i-factor reverse engineering — where you're given the observed property (ΔTf, ΔTb, or π) and must work back to the degree of dissociation, molar mass, or coordination sphere. Colligative properties are 54.5% of the chapter.
Q3: Why is the van't Hoff factor of benzoic acid in benzene 0.5 and not 2?
Because benzoic acid associates into dimers in benzene rather than dissociating. Association uses i = 1 + (1/n − 1)α, so for 100% dimerisation (n=2), i = 0.5. Students who apply the dissociation formula i = 1 + (n−1)α get 2 and land on a designed distractor. A carboxylic acid in a non-polar solvent signals association.
Q4: What are the most common NVQ traps in JEE Main Solutions?
Forgetting to convert Celsius to Kelvin in π = iCRT; not converting kbar to bar in Henry's law (off by 1000×); using the dissociation formula for an associating solute; answering with the liquid mole fraction when the vapour-phase fraction is asked; and assuming density = 1 g/mL when a density is explicitly given. Each is a −1 under the compulsory NVQ rule.
Q5: Are there actual JEE Main Solutions PYQs to practice?
Yes — this article contains 15 representative JEE Main PYQs with worked solutions and traps explained, including Numerical Value type. For the full set of 200+ JEE Main Solutions PYQs mapped across all shifts with TarQ teaching and a Mistake Book, download Logic Bloom. Free to start.