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 13) 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?
Answer: C. An ideal solution has no enthalpy or volume change on mixing (ΔH_mix = 0, ΔV_mix = 0), as stated in NCERT Class 12 Chemistry Chapter 1, page 13.
Why A is wrong: A is wrong because ΔH_mix > 0 indicates an endothermic mixing process, characteristic of positive deviation from ideality, not an ideal solution.
Why B is wrong: B is wrong because ΔV_mix > 0 means volume expansion on mixing, which occurs in non-ideal solutions with positive deviation.
Why D is wrong: D is wrong because ideal solutions obey Raoult's law at all compositions and do not form azeotropes. Azeotropes arise from non-ideal behaviour.
An ethanol–acetone mixture shows a total vapour pressure higher than predicted by Raoult's law. This is an example of:
Answer: D. When observed VP exceeds the Raoult's law prediction, A–B interactions are weaker than A–A and B–B interactions, which defines positive deviation (NCERT Class 12 Chemistry Chapter 1, page 13).
Why A is wrong: A is wrong because an ideal solution would have observed VP exactly matching Raoult's law prediction, not exceeding it.
Why B is wrong: B is wrong because negative deviation means observed VP is lower than Raoult's prediction — the opposite of what is described here.
Why C is wrong: C is wrong because positive deviation, if extreme enough to form an azeotrope, produces a minimum-boiling azeotrope, not a maximum-boiling one.
Which pair of liquids is most likely to form an ideal solution?
Answer: A. Benzene and toluene are structurally similar non-polar hydrocarbons with nearly identical intermolecular forces, forming a near-ideal solution (NCERT Class 12 Chemistry Chapter 1, page 13).
Why B is wrong: B is wrong because chloroform and acetone form a hydrogen bond (C–H⋯O=C) not present in the pure liquids, giving negative deviation.
Why C is wrong: C is wrong because water and ethanol have very different intermolecular forces (water has strong H-bonding networks disrupted by ethanol), leading to positive deviation.
Why D is wrong: D is wrong because HCl is a strong acid that dissociates and interacts strongly with water — far from ideal behaviour.
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:
Answer: B. Using Raoult's law: p = p°_A × x_A + p°_B × x_B = 300 × 0.40 + 500 × 0.60 = 120 + 300 = 420 mmHg (NCERT Class 12 Chemistry Chapter 1).
Why A is wrong: A is wrong because 800 mmHg is simply p°_A + p°_B — adding pure vapour pressures without weighting by mole fractions is incorrect.
Why C is wrong: C is wrong because 380 mmHg results from accidentally swapping the mole fractions: 300 × 0.60 + 500 × 0.40 = 180 + 200 = 380.
Why D is wrong: D is wrong because 200 mmHg appears to come from multiplying only one component (500 × 0.40 = 200) and ignoring the other entirely.
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:
Answer: D. Moles of ethanol = 46/46 = 1 mol; moles of water = 18/18 = 1 mol. x_ethanol = 1/(1+1) = 0.50. This is a mole fraction calculation, not a mass fraction (trap: mole vs mass fraction confusion).
Why A is wrong: A is wrong because 0.46 treats the mass in grams as if it were the mole fraction directly, which has no chemical basis.
Why B is wrong: B is wrong because 0.72 ≈ 46/64 — this is the mass fraction of ethanol (mass of ethanol / total mass), not the mole fraction. Raoult's law requires mole fractions (trap: mole vs mass fraction).
Why C is wrong: C is wrong because 0.28 ≈ 18/64 — this is the mass fraction of water, again confusing mass fraction with mole fraction.
A binary liquid mixture shows negative deviation from Raoult's law. Which statement is correct?
Answer: A. Negative deviation means A–B forces are stronger than those in pure components, making molecules harder to vaporise. Extreme negative deviation produces a maximum-boiling azeotrope (e.g., HNO₃ + water at 393.5 K), per NCERT Class 12 Chemistry Chapter 1.
Why B is wrong: B is wrong because negative deviation involves stronger A–B interactions, releasing energy on mixing, so ΔH_mix < 0 (exothermic).
Why C is wrong: C is wrong because weaker A–B interactions characterise positive deviation, not negative.
Why D is wrong: D is wrong because in negative deviation, the observed VP is lower than the Raoult's law prediction, not higher.
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:
Answer: B. Moles of A = 78/78 = 1 mol; moles of B = 184/92 = 2 mol. x_A = 1/3, x_B = 2/3. p = 120 × (1/3) + 60 × (2/3) = 40 + 40 = 80 mmHg. Two steps: convert mass → moles → mole fractions, then apply Raoult's law.
Why A is wrong: A is wrong because 100 mmHg results from swapping the mole fractions (x_A = 2/3, x_B = 1/3): 120 × 2/3 + 60 × 1/3 = 80 + 20 = 100 mmHg. The larger mole fraction belongs to B, of which there are 2 mol.
Why C is wrong: C is wrong because 90 mmHg = (120 + 60)/2, which assumes equal mole fractions (x = 0.5 each). But x_A = 1/3 and x_B = 2/3 — equal masses do not mean equal moles when molar masses differ.
Why D is wrong: D is wrong because 72 mmHg might arise from accidentally using x_A = 0.2 and x_B = 0.8, perhaps from a ratio error in mole calculation.
For an ideal solution of two volatile components, a plot of total vapour pressure (p) vs. mole fraction of component A (x_A) is:
Answer: C. For an ideal solution, p = p°_B + (p°_A − p°_B) x_A, which is linear in x_A. The total VP varies linearly from p°_B (at x_A = 0) to p°_A (at x_A = 1). This is the graphical statement of Raoult's law for ideal solutions (NCERT Class 12 Chemistry Chapter 1, page 10).
Why A is wrong: A is wrong because a maximum in the p vs. x_A plot indicates positive deviation from Raoult's law — the solution is non-ideal.
Why B is wrong: B is wrong because a minimum in the p vs. x_A plot indicates negative deviation from Raoult's law — also non-ideal behaviour.
Why D is wrong: D is wrong because a horizontal line would mean VP is independent of composition, which only occurs if both pure components have the same vapour pressure — not the general case.
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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.)
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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. 13More in Solutions: 4 exam traps and mistakes · 8 formulas · 2 question patterns from its other lessons.
3 questions from NEET 2020, 2025, 2026. Answers verified against NTA official keys.
The mixture which shows positive deviation from Raoult’s law is :
Recurring question shapes from past papers. Each pattern shows why wrong options look tempting.
uses mass fractions not mole
Substitutes mass fraction for x
Page numbers are the ones printed in the current NCERT textbook (2023 rationalised edition), unless marked pre-2023. The books are free at ncert.nic.in.
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