Subtopics - Solution (NEET)
Six topic blocks: vapour pressure fundamentals and concentration expressions, Raoult's Law with ideal and non-ideal solutions, azeotropes (minimum and maximum boiling point types), four colligative properties with derivations and molecular mass determination, osmotic pressure via Van't Hoff equation, and the Van't Hoff factor for abnormal molecular masses from association or dissociation.
1) Vapour Pressure
Defines vapour pressure as the equilibrium pressure exerted by vapours on the walls of a closed container at a given temperature. Vapour pressure depends only on temperature, not on the volume or surface area of the container. Covers concentration expressions: mass percentage, volume percentage, molality, molarity, normality, mole fraction, and ppm.
2) Azeotropes or Azeotropic mixture
Liquid mixtures that distil without change in composition are azeotropes (constant boiling mixtures). Two types exist: minimum boiling point azeotropes formed by solutions showing positive deviation from Raoult's Law, and maximum boiling point azeotropes formed by solutions showing negative deviation. Cannot be separated by simple distillation.
3) Raoult's Law
States that the partial vapour pressure of any component in a solution is directly proportional to its mole fraction: PA = XA times PA-star. Combined with Dalton's Law: PT = PB-star plus (PA-star minus PB-star) times XA. Ideal solutions obey Raoult's Law at all compositions with delta-H-mixing = 0 and delta-V-mixing = 0. Non-ideal solutions show positive deviation (VP higher than predicted, delta-H > 0) or negative deviation (VP lower than predicted, delta-H < 0).
4) Colligative Properties
Properties of dilute solutions that depend only on the number of solute particles, not their identity. Four types: lowering of vapour pressure (relative lowering = mole fraction of solute), elevation in boiling point (delta-Tb = Kb times m), depression in freezing point (delta-Tf = Kf times m), and osmotic pressure. Each property enables experimental determination of solute molecular mass.
5) Abnormal Molecular Mass and Van't Hoff Factor
When solutes associate or dissociate in solution, colligative properties give abnormal molecular masses. Van't Hoff factor i = observed colligative property / theoretical colligative property. For dissociating solutes i > 1 (more particles). For associating solutes i < 1 (fewer particles). Modified formulas: delta-Tb = i Kb m; delta-Tf = i Kf m; pi = icRT.
6) Osmotic Pressure
Osmosis is the flow of solvent through a semi-permeable membrane from higher solvent concentration (pure solvent) to lower solvent concentration (solution). Osmotic pressure pi = (n/V)RT = cRT, where c is molar concentration. Preferred method for determining molecular mass of macromolecules (proteins, polymers) because osmotic pressure is measurable even for dilute macromolecular solutions where other colligative effects are too small.
Solution Download Notes & Weightage Plan
For each topic in the Solution chapter below, you get (2) the exact resources to download and how to use them, and (3) a simple importance & time plan so NEET students know what to do first and what to revise last.
Foundation topic: defines vapour pressure, its temperature dependence, and all concentration units (molality, molarity, normality, mole fraction, ppm) used throughout the chapter.
1) Download Packs For This Topic (And How To Use Them)
Don't download everything and forget it. Use these like a small "attack kit": read → highlight → test → revise the same sheet again.
2) Importance, Weightage & Time Allocation (Practical)
Use this to avoid over-studying. This topic is usually low effort, quick return if your recall is clean.
- Scoring Focus: Molality versus molarity distinction. Every concentration conversion problem uses this. Know that molality is temperature-independent (mass-based) while molarity changes with temperature (volume-based).
- High-risk Area: Using litres of solution when the formula requires kg of solvent, or vice versa. NEET distractors are designed precisely around this confusion.
- Best Practice Style: For every numerical, first identify whether the problem gives you solvent mass or solution volume, then pick the correct concentration unit before substituting.
Azeotropes or Azeotropic mixture
Constant boiling mixtures that cannot be separated by distillation; directly linked to positive and negative deviation from Raoult's Law.
1) Download Packs For This Topic (And How To Use Them)
Don't download everything and forget it. Use these like a small "attack kit": read → highlight → test → revise the same sheet again.
2) Importance, Weightage & Time Allocation (Practical)
Use this to avoid over-studying. This topic is usually low effort, quick return if your recall is clean.
- Scoring Focus: Link the sign of delta-H-mixing to deviation type: positive delta-H means positive deviation means minimum boiling azeotrope. Negative delta-H means negative deviation means maximum boiling azeotrope.
- High-risk Area: Confusing which deviation type gives which azeotrope. Positive deviation gives MINIMUM boiling point (not maximum) because higher VP means lower boiling point.
- Best Practice Style: Mnemonic: Positive deviation = More VP = Less BP needed = Minimum boiling azeotrope. The chain is P-M-L-Min.
The central law of liquid-liquid solutions: PA = XA times PA-star. Combined with Dalton's Law for total vapour pressure. Defines ideal and non-ideal solutions.
1) Download Packs For This Topic (And How To Use Them)
Don't download everything and forget it. Use these like a small "attack kit": read → highlight → test → revise the same sheet again.
2) Importance, Weightage & Time Allocation (Practical)
Use this to avoid over-studying. This topic is usually low effort, quick return if your recall is clean.
- Scoring Focus: Total pressure calculation from mole fraction data: PT = PB-star + (PA-star minus PB-star) times XA. Know the three criteria for ideal solution: Raoult's Law obeyed, delta-H = 0, delta-V = 0.
- High-risk Area: Forgetting that mole fractions in vapour phase differ from those in liquid phase. NEET sometimes asks for vapour-phase mole fraction using yA = PA/PT.
- Best Practice Style: For every Raoult's Law problem: (1) write PA = XA PA-star for each component, (2) add to get PT, (3) if asked for vapour composition, calculate yA = PA/PT.
Four solution properties depending only on solute particle count: relative lowering of VP, elevation in boiling point, depression in freezing point. Each with molecular mass determination formula.
1) Download Packs For This Topic (And How To Use Them)
Don't download everything and forget it. Use these like a small "attack kit": read → highlight → test → revise the same sheet again.
2) Importance, Weightage & Time Allocation (Practical)
Use this to avoid over-studying. This topic is usually low effort, quick return if your recall is clean.
- Scoring Focus: Molecular mass determination: MB = (K times WB times 1000)/(delta-T times WA). Whether using Kb or Kf, the formula structure is identical. Depression in freezing point problems dominate because Kf for water is a standard given value (1.86 K kg/mol).
- High-risk Area: The 1000 factor in the MW formula accounts for converting WA from grams to kilograms. Omitting it gives molecular mass off by three orders of magnitude. Also, delta-T is always positive (magnitude of temperature change).
- Best Practice Style: Always write the full formula with units. Confirm that WA is in grams and the factor 1000 is present. Cross-check: if MW comes out in thousands for a simple organic solute, you probably dropped the 1000.
Abnormal Molecular Mass and Van't Hoff Factor
Explains why electrolytes and associating solutes give abnormal colligative effects. Van't Hoff factor i corrects all four colligative property formulas.
1) Download Packs For This Topic (And How To Use Them)
Don't download everything and forget it. Use these like a small "attack kit": read → highlight → test → revise the same sheet again.
2) Importance, Weightage & Time Allocation (Practical)
Use this to avoid over-studying. This topic is usually low effort, quick return if your recall is clean.
- Scoring Focus: Quick calculation of i for common electrolytes (NaCl = 2, BaCl2 = 3, AlCl3 = 4 at full dissociation) and using i in the modified colligative formula. NEET asks: which solution has highest osmotic pressure given equimolar NaCl, glucose, BaCl2 - answer is BaCl2 because i = 3 is highest.
- High-risk Area: Using M (number of ions produced per formula unit) and confusing it with molar mass. In the dissociation formula alpha = (i minus 1)/(M minus 1), M is the count of particles from one formula unit, NOT the molar mass in g/mol.
- Best Practice Style: Before substituting into any Van't Hoff formula, first write the dissociation equation and count the number of ions produced. That count is M. Then i = 1 + alpha(M minus 1) for dissociation.
Osmosis through semi-permeable membranes and the Van't Hoff equation pi = cRT. Preferred for macromolecular mass determination.
1) Download Packs For This Topic (And How To Use Them)
Don't download everything and forget it. Use these like a small "attack kit": read → highlight → test → revise the same sheet again.
2) Importance, Weightage & Time Allocation (Practical)
Use this to avoid over-studying. This topic is usually low effort, quick return if your recall is clean.
- Scoring Focus: Two scoring points: (1) pi = cRT numerical with given concentration and temperature, (2) conceptual question on why osmotic pressure is preferred for macromolecular mass determination (answer: other CPs are too small to measure).
- High-risk Area: Unit mismatch: if pi is in bar, use R = 0.083 L bar/(mol K); if in atm, use R = 0.0821 L atm/(mol K). Using the wrong R value gives wrong molecular mass. Also, V must be in litres, not mL.
- Best Practice Style: Write pi = cRT with units explicitly. Circle the units of pressure and match R accordingly. Convert V to litres and T to Kelvin before substituting.
Solution Chapter NEET Traps & Common Mistakes (Topic-Wise)
Each subtopic below is of the Solution chapter and shows what NEET students usually do wrong in NEET examination, a short example of the mistake, and how NEET frames the question to trick you with close options are given below.
Mistake Snapshot (What Students Do Wrong)
- Using molarity instead of molality in colligative property formulas: Colligative property equations use molality (mol solute / kg solvent). NEET provides data as molarity or mass in solution volume. Substituting molarity directly into delta-Tf = Kf times m gives a wrong answer because molarity uses litres of solution, not kg of solvent.
- Dropping the factor 1000 in molecular mass formula: The formula MB = (Kf times WB times 1000) / (delta-Tf times WA) requires WA in grams. The 1000 converts grams to kilograms. Omitting it gives molecular mass exactly 1000 times too small, which NEET places as a distractor.
A solution contains 6 g of urea (MW = 60) in 500 g of water. Correct molality = (6/60) / (500/1000) = 0.2 m. If you mistakenly use 500 mL of solution as denominator for molarity, you get 0.2 M which happens to match here but fails when density is not 1 g/mL. For a sugar solution with density 1.2 g/mL, the error produces a 20% deviation.
How NEET Frames The Trap
NEET gives solute mass and solvent mass in grams. You must convert solvent mass to kg (divide by 1000) to get molality. Distractors are calculated using solution volume or omitting the 1000 factor.
Q. What is the depression in freezing point when 3 g of urea (MW = 60) is dissolved in 500 g of water? (Kf = 1.86 K kg/mol)
A. 0.186 K B. 1.86 K C. 0.0186 K D. 18.6 K
Trick: m = (3/60) / (500/1000) = 0.1 mol/kg. delta-Tf = 1.86 times 0.1 = 0.186 K (Option A). Option B (1.86) uses m = 1 by forgetting to divide mass by MW. Option C (0.0186) forgets to convert 500 g to 0.5 kg, using 5 instead of 0.5. Option D (18.6) multiplies instead of dividing.
Mistake Snapshot (What Students Do Wrong)
- Forgetting to multiply by i for electrolyte solutes: For electrolytes like NaCl (i = 2), BaCl2 (i = 3), the colligative property formulas must include i: delta-Tf = iKfm, pi = icRT. Using the non-electrolyte formula gives the theoretical value, not the observed value. NEET distractors are computed without i.
- Confusing M (ion count) with molecular mass in the dissociation formula: In alpha = (i minus 1) / (M minus 1), M is the number of particles produced per formula unit on complete dissociation, NOT the molar mass. For NaCl, M = 2 (one Na-plus and one Cl-minus). Plugging in molar mass 58.5 instead of 2 gives an absurdly small alpha.
Osmotic pressure of 0.1 M NaCl at 300K. Without i: pi = 0.1 times 0.0821 times 300 = 2.463 atm. With i = 2: pi = 2 times 2.463 = 4.926 atm. NEET provides 2.463 atm as a distractor (no i correction). Students who forget NaCl is an electrolyte pick the distractor.
How NEET Frames The Trap
NEET identifies the solute as an electrolyte (NaCl, CaCl2, etc.) in the problem. If student does not recognise it as an electrolyte or forgets to apply i, they calculate the non-electrolyte answer which is always one of the four options.
Q. Which 0.1 M aqueous solution will show the highest osmotic pressure at 25 degrees C?
A. Glucose B. NaCl C. BaCl2 D. Urea
Trick: Osmotic pressure pi = icRT. For glucose (i=1): pi = 0.1RT. For NaCl (i=2): pi = 0.2RT. For BaCl2 (i=3): pi = 0.3RT. For urea (i=1): pi = 0.1RT. BaCl2 has highest i = 3, so highest pi. Students who ignore i pick glucose or urea (same pi) and miss BaCl2.
Mistake Snapshot (What Students Do Wrong)
- Confusing which deviation gives which azeotrope type: Positive deviation (VP higher than Raoult prediction) gives MINIMUM boiling point azeotrope. Negative deviation (VP lower) gives MAXIMUM boiling point azeotrope. Students reverse this because positive sounds like it should go with maximum.
- Mixing up liquid-phase and vapour-phase mole fractions: Raoult's Law PA = XA PA-star uses liquid-phase mole fraction XA. The vapour-phase mole fraction yA = PA / PT is different from XA. NEET asks for vapour composition which requires the extra step of dividing PA by PT.
A mixture of A (PA-star = 300 torr) and B (PB-star = 100 torr) with XA = 0.4. PA = 0.4 times 300 = 120 torr; PB = 0.6 times 100 = 60 torr; PT = 180 torr. Vapour mole fraction of A: yA = 120/180 = 0.667 (NOT 0.4). Students who report XA = 0.4 as vapour mole fraction lose the mark.
How NEET Frames The Trap
NEET asks for the mole fraction of component A in the VAPOUR phase above an ideal solution. Students who do not distinguish liquid-phase XA from vapour-phase yA give the wrong answer by reporting XA directly.
Q. An ideal solution contains A (PA-star = 400 mmHg) and B (PB-star = 200 mmHg). If XA = 0.3, what is the total vapour pressure?
A. 260 mmHg B. 300 mmHg C. 200 mmHg D. 400 mmHg
Trick: PT = XA PA-star + XB PB-star = 0.3(400) + 0.7(200) = 120 + 140 = 260 mmHg (Option A). Option B (300) is the arithmetic mean of 400 and 200, ignoring mole fractions. Option C and D are the pure component VPs.
Mistake Snapshot (What Students Do Wrong)
- Using wrong value of R for given pressure units: If osmotic pressure is in atm, R = 0.0821 L atm / (mol K). If in bar, R = 0.083 L bar / (mol K). Using the wrong R value gives molecular mass off by approximately 1.3%. NEET distractors are spaced to catch this error.
- Substituting volume in mL instead of litres: In pi = (n/V)RT, V must be in litres. Substituting 200 mL instead of 0.2 L makes pi 1000 times too small or MW 1000 times too large. This is the most common careless error in osmotic pressure numericals.
1.26 g protein in 200 mL solution at 300 K gives pi = 2.57 times 10 to the power minus 3 bar. MB = (1.26 times 0.083 times 300) / (2.57E-3 times 0.200) = 31.374 / 5.14E-4 = 61038 g/mol. If V = 200 (not 0.200 L), MB = 61.038 g/mol, which is absurdly small for a protein. NEET places this as a distractor.
How NEET Frames The Trap
NEET gives volume in mL and pressure in bar or atm. The student must convert mL to L and match R to the pressure unit before substituting. Both conversions are tested simultaneously.
Q. 200 mL of an aqueous protein solution (1.26 g) at 300 K has osmotic pressure 2.57 x 10^-3 bar. What is the molar mass? (R = 0.083 L bar / mol K)
A. 61038 g/mol B. 61.038 g/mol C. 31011 g/mol D. 122044 g/mol
Trick: MB = wRT / (pi V) = (1.26 x 0.083 x 300) / (2.57E-3 x 0.200) = 61038 g/mol (Option A). Option B (61.038) uses V = 200 instead of 0.2 L. Option C halves the result (wrong factor). Option D doubles it.
Mistake Snapshot (What Students Do Wrong)
- Calculating i > 1 for associating solutes instead of i < 1: Association reduces the number of particles (molecules combine). Therefore i < 1 and observed molecular mass is HIGHER than expected. Students who confuse association with dissociation calculate i > 1, giving a MW lower than expected.
- Using aqueous dissociation formula for non-aqueous association: The formula alpha = (i minus 1)/(M minus 1) is for dissociation. For association: i = 1 minus alpha(1 minus 1/n), where n is the number of molecules combining. Using the wrong formula gives a negative alpha, which is physically meaningless.
Acetic acid (MW = 60) dimerises in benzene (n = 2). If observed MW = 100: i = expected MW / observed MW = 60/100 = 0.6. From i = 1 minus alpha(1 minus 1/2): 0.6 = 1 minus 0.5 alpha, so alpha = 0.8 (80% association). If student uses dissociation formula: alpha = (0.6 minus 1)/(2 minus 1) = minus 0.4, which is nonsensical.
How NEET Frames The Trap
NEET specifies the solvent is benzene or another non-aqueous solvent and the solute is an organic acid. This is the association signal. The observed MW will be higher than the true MW, and i < 1.
Q. Acetic acid (MW = 60) forms a dimer in benzene. If the degree of association is 0.8, what is the Van't Hoff factor?
A. 0.6 B. 1.4 C. 1.8 D. 2.0
Trick: i = 1 minus alpha(1 minus 1/n) = 1 minus 0.8(1 minus 0.5) = 1 minus 0.4 = 0.6 (Option A). Option B (1.4) uses the dissociation formula i = 1 + alpha(n minus 1) = 1 + 0.8(1) = 1.8 (which matches C). Students who forget association gives i < 1 will pick B or C.