The trap that costs marks on ideal vs. non-ideal solution questions is deceptively simple: you're given masses of two liquids and asked to find the total vapour pressure. You plug the masses straight into Raoult's law as if they were mole fractions. They aren't. Raoult's law demands mole fractions, not mass fractions — and confusing the two produces a wrong answer that often appears as a distractor.
What makes a solution ideal? NCERT Class 12 Chemistry Chapter 1 (page 10) defines an ideal solution as one where the intermolecular forces between solute–solvent, solute–solute, and solvent–solvent are nearly identical. In such a solution, each component obeys Raoult's law over the entire composition range:
p = p₁° x₁ + p₂° x₂
where p₁° and p₂° are vapour pressures of the pure components and x₁, x₂ are their mole fractions.
Key signatures of an ideal solution: ΔH_mix = 0, ΔV_mix = 0. Examples: benzene + toluene, n-hexane + n-heptane.
Non-ideal solutions deviate because intermolecular forces change on mixing:
NEET connection: Questions test whether you can (a) calculate total VP from mole fractions using Raoult's law, and (b) classify a system as positive or negative deviation from given data. The mole-vs-mass fraction trap appears as a distractor in Raoult's law VP calculations — a common source of negative marks.
Watch out: When a problem gives component amounts in grams, always convert to moles first. The number you divide by total moles is the mole fraction — never the mass ratio.
Select an option to see the explanation. Wrong answers show why your choice was tempting — and name the exact trap it exploits.
Which of the following is a characteristic of an ideal solution?
An ethanol–acetone mixture shows a total vapour pressure higher than predicted by Raoult's law. This is an example of:
Which pair of liquids is most likely to form an ideal solution?
A solution of two volatile liquids A and B has p°_A = 300 mmHg and p°_B = 500 mmHg. If x_A = 0.40, the total vapour pressure of the ideal solution is:
46 g of ethanol (M = 46 g/mol) and 18 g of water (M = 18 g/mol) are mixed. The mole fraction of ethanol in the solution is:
A binary liquid mixture shows negative deviation from Raoult's law. Which statement is correct?
Two liquids A (M_A = 78 g/mol, p°_A = 120 mmHg) and B (M_B = 92 g/mol, p°_B = 60 mmHg) form an ideal solution. If 78 g of A and 184 g of B are mixed, the total vapour pressure is:
For an ideal solution of two volatile components, a plot of total vapour pressure (p) vs. mole fraction of component A (x_A) is:
Given
Two volatile liquids P and Q form an ideal solution. p°_P = 150 mmHg, p°_Q = 350 mmHg. M_P = 60 g/mol, M_Q = 40 g/mol. A solution contains 120 g of P and 80 g of Q.
Required
Total vapour pressure of the solution.
Concept
For an ideal solution, Raoult's law states that total VP is the sum of each component's pure VP weighted by its mole fraction. The mole fraction must be calculated from moles — not from mass.
Formula
p = p°_P × x_P + p°_Q × x_Q
Substitution setup
Moles of P = 120/60 = 2 mol Moles of Q = 80/40 = 2 mol Total moles = 4 mol x_P = 2/4 = 0.50, x_Q = 2/4 = 0.50 p = 150 × 0.50 + 350 × 0.50
Calculation
p = 75 + 175 = 250 mmHg Note: the molar masses (60 g/mol, 40 g/mol) are exact problem-defined values and do not limit significant figures.
Final answer
Total vapour pressure = 250 mmHg
Common trap
If you used mass fractions instead of mole fractions: mass fraction of P = 120/200 = 0.60, mass fraction of Q = 0.40. Then p = 150 × 0.60 + 350 × 0.40 = 90 + 140 = 230 mmHg — a wrong answer that would appear as a distractor. The error: Raoult's law uses mole fractions, not mass fractions.
Similar NEET-style question
Benzene (M = 78 g/mol, p° = 100 mmHg) and toluene (M = 92 g/mol, p° = 40 mmHg) form an ideal solution. If 39 g of benzene is mixed with 46 g of toluene, find the total vapour pressure. (Answer: moles benzene = 0.50, moles toluene = 0.50; x = 0.50 each; p = 100 × 0.50 + 40 × 0.50 = 70 mmHg.) ---
Ideal: obeys Raoult's law over all composition; ΔH_mix = 0, ΔV_mix = 0 (e.g. n-hexane + n-heptane). Positive deviation: A-B interactions weaker than A-A, B-B (more volatile). Negative deviation: A-B stronger (less volatile).
-- NCERT Class 12 Chemistry, Ch. 1, p. 10Molal concentration: moles of solute per kg of solvent. Temperature-independent.
| Symbol | Quantity | SI Unit |
|---|---|---|
| m | molality | mol/kg |
| n | moles solute | mol |
Molar concentration: moles of solute per litre of solution.
| Symbol | Quantity | SI Unit |
|---|---|---|
| M | molarity | mol/L |
| n | moles solute | mol |
| V | solution volume | L |
Solute raises boiling point. K_b is ebullioscopic constant of solvent (water: 0.52 K kg/mol).
| Symbol | Quantity | SI Unit |
|---|---|---|
| ΔT_b | BP elevation | K |
| K_b | ebullioscopic constant | K kg/mol |
| m | molality | mol/kg |
Solute lowers freezing point. K_f is cryoscopic constant of solvent (water: 1.86 K kg/mol). Used for molar mass determination.
| Symbol | Quantity | SI Unit |
|---|---|---|
| ΔT_f | FP depression | K |
| K_f | cryoscopic constant | K kg/mol |
| m | molality | mol/kg |
Pressure required to prevent osmosis. C in mol/L; T in K. Used for high-molar-mass biomolecules.
| Symbol | Quantity | SI Unit |
|---|---|---|
| π | osmotic pressure | Pa |
| C | molarity | mol/L |
| R | gas constant | J/mol/K |
| T | temp | K |
Total vapor pressure of ideal solution = sum of mole-fraction-weighted vapor pressures of components.
| Symbol | Quantity | SI Unit |
|---|---|---|
| p | total vapor pressure | Pa |
| p_i° | pure component vp | Pa |
| x_i | mole fraction | - |
For non-volatile solute: relative lowering of VP equals mole fraction of solute.
| Symbol | Quantity | SI Unit |
|---|---|---|
| p | solution vp | Pa |
| p° | pure solvent vp | Pa |
| x_solute | mole fraction | - |
Correction factor for electrolytes. NaCl: i≈2; CaCl₂: i≈3. Multiply colligative formula by i.
| Symbol | Quantity | SI Unit |
|---|---|---|
| i | Van't Hoff factor | - |
These are the exact patterns that cause wrong answers in NEET. Each trap includes when it triggers and how to avoid it.
Category: Similar Terms
Student uses mass fraction (w₁/total mass) where mole fraction (n₁/total moles) is required.
Question gives masses or molar masses and asks about Raoult's law or vapor pressure.
Raoult's law uses MOLE fractions, not mass fractions. Convert mass to moles first using molar mass.
Category: Similar Terms
Student uses non-electrolyte colligative formula for ionic compound. NaCl: i ≈ 2; CaCl₂: i ≈ 3.
Question gives an ionic compound (NaCl, CaCl₂, K₂SO₄) and asks for colligative property.
For electrolytes, multiply colligative formula by Van't Hoff factor i. NaCl → Na⁺ + Cl⁻ (i=2). CaCl₂ → Ca²⁺ + 2Cl⁻ (i=3). K₂SO₄ → 2K⁺ + SO₄²⁻ (i=3).
Root cause: formula misuse
Convert mass to moles first using molar mass. Raoult uses MOLE fractions.
Root cause: formula misuse
For NaCl i ≈ 2, CaCl₂ i ≈ 3, K₂SO₄ i ≈ 3. Multiply colligative formulas by i.
9 questions from NEET 2021, 2022, 2023, 2024, 2025. Answers verified against NTA official keys.
Which of the following aqueous solution will exhibit highest boiling point?
The IUPAC name of an element with atomic number 119 is
The structures of beryllium chloride in solid state and vapour phase, are :
Recurring question shapes from past papers. Each pattern shows why wrong options look tempting.
forgets i factor electrolytes
Uses non-electrolyte formula for ionic solute
forgets conversion of units
Mixes Pa with atm; L with m^3
uses mass fractions not mole
Substitutes mass fraction for x
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