Elevation Boiling Point

8 MCQs3 revision cards9-step worked example

Source: NCERT SolutionsPYQ coverage: NEET 2021, 2022, 2023, 2024, 2025Official key: NTA-verifiedLast reviewed: May 2026

Lesson

When you dissolve a non-volatile solute in a solvent, the boiling point of the solution is higher than that of the pure solvent. This is boiling-point elevation — a colligative property that depends only on the number of solute particles, not their identity.

The trap that costs marks: an ionic compound like NaCl dissociates into ions. If a question hands you NaCl dissolved in water and you plug molality straight into ΔT_b = K_b · m, you get the answer for a non-electrolyte. The actual elevation is larger because NaCl produces two ions per formula unit — you must multiply by the Van 't Hoff factor i. Forgetting i is a common confusion in NEET colligative-property calculations.

The core relationship (NCERT Class 12 Chemistry Chapter 1, page 18):

ΔT_b = i · K_b · m

where ΔT_b is the elevation in boiling point, K_b is the ebullioscopic constant of the solvent (for water, K_b = 0.52 K·kg/mol), m is molality (moles of solute per kg of solvent), and i is the Van 't Hoff factor.

For non-electrolytes (glucose, urea), i = 1 and the formula reduces to ΔT_b = K_b · m. For electrolytes: NaCl → Na⁺ + Cl⁻ gives i ≈ 2; CaCl₂ → Ca²⁺ + 2Cl⁻ gives i ≈ 3.

Key points to lock in:

Molality, not molarity — colligative formulas use moles of solute per kg of solvent, not per litre of solution. Molality is temperature-independent.
K_b is a solvent property. Water's K_b = 0.52 K·kg/mol. Different solvents have different K_b values.
Higher ΔT_b means more solute particles. Comparing 0.1 m NaCl (i = 2) vs 0.1 m glucose (i = 1): NaCl gives double the elevation.
Boiling-point elevation can be used to determine molar mass of an unknown non-electrolyte solute: M₂ = (i · K_b · w₂ × 1000) / (ΔT_b · w₁), where w₂ is mass of solute and w₁ is mass of solvent in grams.

Watch out: when the question says "0.1 molal NaCl," the molality refers to the formula units dissolved, not the total ion concentration. The i factor handles the ion count separately.

Practice MCQs

Select an option to see the explanation. Wrong answers show why your choice was tempting — and name the exact trap it exploits.

MCQ 1Easy RecallPractice

The boiling-point elevation of a solution depends on:

MCQ 2Easy RecallPractice

The ebullioscopic constant K_b depends on:

MCQ 3Easy RecallPractice

Which of the following is used as the unit of molality?

MCQ 4Direct ApplicationPractice

3.0 g of urea (molar mass = 60 g/mol) is dissolved in 500 g of water. The boiling-point elevation is (K_b for water = 0.52 K·kg/mol):

MCQ 5Direct ApplicationPractice

The Van 't Hoff factor *i* for K₂SO₄, assuming complete dissociation, is:

MCQ 6Direct ApplicationPractice

0.1 molal aqueous solutions of NaCl and glucose are prepared. Which has a higher boiling point?

MCQ 7CalculationPractice

5.85 g of NaCl (molar mass = 58.5 g/mol) is dissolved in 250 g of water. Assuming complete dissociation, the boiling-point elevation is (K_b for water = 0.52 K·kg/mol):

MCQ 8CalculationPractice

An aqueous solution of a non-electrolyte solute boils at 100.26°C. If K_b for water is 0.52 K·kg/mol, the molality of the solution is:

Quick recall before you leave

Worked Example

1
Given
- Mass of urea (w₂) = 6.0 g - Molar mass of urea (M₂) = 60 g/mol (exact, problem-defined) - Mass of solvent (w₁) = 200 g = 0.200 kg - K_b for water = 0.52 K·kg/mol - Urea is a non-electrolyte → *i* = 1
2
Required
Boiling point of the solution (T_b).
3
Concept
Boiling-point elevation is a colligative property. A non-volatile solute raises the boiling point by ΔT_b = i · K_b · m. For non-electrolytes, *i* = 1.
4
Formula
ΔT_b = i · K_b · m, where m = n / (mass of solvent in kg)
5
Substitution
Moles of urea: n = 6.0 / 60 = 0.10 mol Molality: m = 0.10 / 0.200 = 0.50 mol/kg ΔT_b = 1 × 0.52 × 0.50
6
Calculation
ΔT_b = 0.52 × 0.50 = 0.26 K Note: the molar mass 60 g/mol and the integer 1 (Van 't Hoff factor for a non-electrolyte) are exact values — they do not limit significant figures. The given data (6.0 g, 200 g, 0.52 K·kg/mol) are all 2 significant figures, so the answer is reported to 2 significant figures.
7
Final answer
T_b = 100.00 + 0.26 = **100.26°C** The boiling point of the solution is 100.26°C (or equivalently, ΔT_b = 0.26 K).
8
Common trap
If the solute were NaCl instead of urea at the same molality, forgetting to multiply by *i* = 2 would give exactly half the correct ΔT_b. For any ionic solute, always ask: "How many ions does one formula unit produce?"
9
Similar NEET-style question
"0.5 molal aqueous solution of CaCl₂ is prepared. Calculate the elevation in boiling point. Assume complete dissociation. (K_b = 0.52 K·kg/mol)." Approach: CaCl₂ → Ca²⁺ + 2Cl⁻, so *i* = 3. ΔT_b = 3 × 0.52 × 0.5 = 0.78 K. ---

Before solving, remember these

Formula

Boiling point elevation

ΔT_b = K_b × m, where K_b is ebullioscopic constant of solvent (water: 0.52 K·kg/mol), m is molality.

-- NCERT Class 12 Chemistry, Ch. 1, p. 18

Formulas

Molality

m = n / m_{solv}

Molal concentration: moles of solute per kg of solvent. Temperature-independent.

Symbol	Quantity	SI Unit
m	molality	mol/kg
n	moles solute	mol

Valid when

Mass of SOLVENT (not solution)

Molarity

M = n / V

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

Valid when

Volume of SOLUTION not solvent
Temperature dependent (volume changes with T)

Boiling-point elevation

Δ T_{b} = K_{b} m

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

Valid when

Dilute solution
Non-electrolyte

Freezing-point depression

Δ T_{f} = K_{f} m

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

Valid when

Dilute solution
Non-electrolyte (else multiply by i)

Osmotic pressure

π = C R T

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

Valid when

Dilute solution
Semipermeable membrane separating pure solvent from solution

Raoult's law

p = p_{1}^{\circ} x_{1} + p_{2}^{\circ} x_{2}

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	-

Valid when

Ideal solution
Both volatile

Relative lowering of VP

\frac{p ^{\circ} - p}{p ^{\circ}} = x_{solute}

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	-

Valid when

Non-volatile solute
Dilute solution
Non-electrolyte (else use i)

Van't Hoff factor

i = \frac{observed}{theoretical}

Correction factor for electrolytes. NaCl: i≈2; CaCl₂: i≈3. Multiply colligative formula by i.

Symbol	Quantity	SI Unit
i	Van't Hoff factor	-

Valid when

Electrolyte solution
Account for ion-pair association/dissociation

Exam Traps & Common Mistakes

These are the exact patterns that cause wrong answers in NEET. Each trap includes when it triggers and how to avoid it.

Trap

Mole vs mass fraction

Category: Similar Terms

Student uses mass fraction (w₁/total mass) where mole fraction (n₁/total moles) is required.

When it triggers

Question gives masses or molar masses and asks about Raoult's law or vapor pressure.

How to avoid

Raoult's law uses MOLE fractions, not mass fractions. Convert mass to moles first using molar mass.

Trap

Forgetting Van't Hoff factor

Category: Similar Terms

Student uses non-electrolyte colligative formula for ionic compound. NaCl: i ≈ 2; CaCl₂: i ≈ 3.

When it triggers

Question gives an ionic compound (NaCl, CaCl₂, K₂SO₄) and asks for colligative property.

How to avoid

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).

Mistake

Substitutes mass fraction for mole fraction in Raoult's law.

Root cause: formula misuse

Correction

Convert mass to moles first using molar mass. Raoult uses MOLE fractions.

Mistake

Forgets Van't Hoff factor i for ionic compounds in colligative formulas.

Root cause: formula misuse

Correction

For NaCl i ≈ 2, CaCl₂ i ≈ 3, K₂SO₄ i ≈ 3. Multiply colligative formulas by i.

Past Year Questions

9 questions from NEET 2021, 2022, 2023, 2024, 2025. Answers verified against NTA official keys.

NEET 2025

Which of the following aqueous solution will exhibit highest boiling point?

10.015M C H O

20.01M Urea 6 12 6

30.01M KNO

40.01M Na SO 3 2 4

NTA Answer: Option 4(final)

NEET 2024Revised key

The Henry’s law constant (K ) values of three gases (A, B, C) in water are 145, 2 × 10–5 and 35 kbar, H respectively. The solubility of these gases in water follow the order:

1B > A > C

2B > C > A

3A > C > B

4A > B > C

NTA Answer: Option 2(revised_final)

NEET 2024Revised key

Given below are two statements: Statement I: The boiling point of hydrides of Group 16 elements follow the order H O > H Te > H Se > H S. 2 2 2 2 Statement II: On the basis of molecular mass, H O is expected to have lower boiling point than the other 2 members of the group but due to the presence of extensive H-bonding in H O, it has higher boiling point. 2 In the light of the above statements, choose the correct answer from the options given below:

1Both Statement I and Statement II are true

2Both Statement I and Statement II are false

3Statement I is true but Statement II is false

4Statement I is false but Statement II is true

NTA Answer: Option 1(revised_final)

NEET 2024Revised key

The plot of osmotic pressure () vs concentration (mol L–1) for a solution gives a straight line with slope 25.73 L bar mol–1. The temperature at which the osmotic pressure measurement is done is (Use R = 0.083 L bar mol–1 K–1)

137°C

2310°C

325.73°C

412.05°C

NTA Answer: Option 1(revised_final)

NEET 2023

Given below are two statements : one is labelled as Assertion A and the other is labelled as Reason R Assertion A : Helium is used to dilute oxygen in diving apparatus. Reason R : Helium has high solubility in O . 2 In the light of the above statements, choose the correct answer from the options given below

1Both A and R are true and R is NOT the correct explanation of A

2A is true but R is false

3A is false but R is true

4Both A and R are true and R is the correct explanation of A

NTA Answer: Option 2(final)

NEET 2022

The IUPAC name of an element with atomic number 119 is

1ununoctium

2ununennium

3unnilennium

4unununnium

NTA Answer: Option 2(final)

NEET 2021

The structures of beryllium chloride in solid state and vapour phase, are :

1Chain in both

2Chain and dimer, respectively

3Linear in both

4Dimer and Linear, respectively

NTA Answer: Option 2(final)

NEET 2021

A particular station of All India Radio, New Delhi broadcasts on a frequency of 1,368 kHz (kilohertz). The wavelength of the electromagnetic radiation emitted by the transmitter is : [speed of light 108 ms–1] c = 3.0 ×

121.92 cm

2219.3 m

3219.2 m

42192 m

NTA Answer: Option 2(final)

NEET 2021

The following solutions were prepared by dissolving 10 g of glucose (C H O ) in 250 ml of water (P ), 6 12 6 1 10 g of urea (CH N O) in 250 ml of water (P ) and 4 2 2 10 g of sucrose (C H O ) in 250 ml of water (P ). 12 22 11 3 The right option for the decreasing order of osmotic pressure of these solutions is :

1P > P > P 3 1 2

2P > P > P 2 1 3

3P > P > P 1 2 3

4P > P > P 2 3 1

NTA Answer: Option 2(final)

How NEET usually asks this

Recurring question shapes from past papers. Each pattern shows why wrong options look tempting.

Calculate ΔT_f or ΔT_b given molality. Or determine molar mass from observed colligative property.

Multi StepMedium

Common distractors

forgets i factor electrolytes

Uses non-electrolyte formula for ionic solute

π = CRT. Find molar mass of solute from observed osmotic pressure.

Direct ApplicationEasy

Common distractors

forgets conversion of units

Mixes Pa with atm; L with m^3

Find total VP of ideal solution from mole fractions and pure-component VPs.

Direct ApplicationEasy

Common distractors

uses mass fractions not mole

Substitutes mass fraction for x

Sources

NCERT refs: Class 12 Chemistry Chapter 1, p.18

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Elevation Boiling Point

Lesson

Practice MCQs

Quick recall before you leave

ΔT_b = i · K_b · m

m = (moles solute) / (kg solvent)

i = observed / theoretical

Worked Example

Before solving, remember these

Formulas

Valid when

Valid when

Valid when

Valid when

Valid when

Valid when

Valid when

Valid when

Exam Traps & Common Mistakes

When it triggers

How to avoid

When it triggers

How to avoid

Correction

Correction

Past Year Questions

How NEET usually asks this

Common distractors

Common distractors

Common distractors

Sources

Free NEET study resources