Concentration Units

8 MCQs2 revision cards9-step worked example

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

Lesson

The topic "Methods of expressing concentration" is deceptively simple — it is the gateway skill for every colligative-property calculation in NEET Chapter 5. The common failure mode is not that aspirants forget the formulas, but that they confuse the denominator: solvent vs. solution, mass vs. volume, and when each unit is appropriate.

NCERT Class 12 Chemistry Chapter 1, page 4, defines a solution as a homogeneous mixture of two or more substances. Concentration describes how much solute is present relative to the solution or solvent. NEET expects fluency in these seven concentration expressions:

Mass percentage (w/w%): (mass of solute / mass of solution) × 100. Independent of temperature.

Volume percentage (v/v%): (volume of solute / volume of solution) × 100. Temperature-dependent.

Mass by volume percentage (w/v%): (mass of solute in g / volume of solution in mL) × 100. Common in pharmacy.

Parts per million (ppm): (mass of solute / mass of solution) × 10⁶. Used for trace quantities — pollutant concentrations, dissolved oxygen.

Mole fraction (x): x₁ = n₁ / (n₁ + n₂). Dimensionless. x₁ + x₂ = 1 for a binary solution. Used in Raoult's law and vapour-pressure calculations (topics covered in separate lessons).

Molarity (M): moles of solute per litre of solution. Temperature-dependent because volume changes with temperature.

Molality (m): moles of solute per kilogram of solvent. Temperature-independent because mass does not change with temperature.

The critical distinction: Molarity uses solution volume; molality uses solvent mass. NEET questions routinely test whether you substitute the correct denominator. A second high-frequency confusion: mole fraction vs. mass fraction — these are not interchangeable and require molar-mass conversion between them.

Watch out: when a problem gives you mass of solution (not solvent), you must subtract the solute mass before computing molality.

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

Which of the following concentration terms is independent of temperature?

MCQ 2Easy RecallPractice

The molality of a solution is defined as the number of moles of solute dissolved in:

MCQ 3Easy RecallPractice

For a binary solution, if the mole fraction of solute is 0.2, the mole fraction of solvent is:

MCQ 4Direct ApplicationPractice

5.85 g of NaCl (molar mass = 58.5 g/mol) is dissolved in 500 g of water. The molality of the solution is:

MCQ 5Direct ApplicationPractice

A solution is prepared by dissolving 18 g of glucose (molar mass = 180 g/mol) in 178.2 g of water to make 200 mL of solution. Which pair of values is correct? (i) Molality (ii) Molarity

MCQ 6Direct ApplicationPractice

A solution contains 10 g of solute in 90 g of water. The mass percentage of the solute is:

MCQ 7CalculationPractice

A 1.0 molal aqueous solution of urea (molar mass = 60 g/mol) is prepared. The mass of solution that contains 1 mole of urea is:

MCQ 8CalculationPractice

0.5 moles of ethanol (C₂H₅OH, molar mass = 46 g/mol) is mixed with 2.5 moles of water (molar mass = 18 g/mol). The mole fraction of ethanol and the molality of ethanol in this solution are, respectively:

Quick recall before you leave

Worked Example

1
Given
- Mass of glucose = 36 g - Molar mass of glucose = 180 g/mol (exact, defined by molecular formula) - Mass of solvent (water) = 500 g - Density of solution = 1.072 g/mL
2
Required
(a) Molality (m) in mol/kg (b) Molarity (M) in mol/L
3
Concept
Molality uses solvent mass in the denominator; molarity uses solution volume. For molarity, we need the solution volume, which requires knowing the total solution mass and its density.
4
Formula
- m = n / m_solvent (kg) - M = n / V_solution (L)
5
Substitution
- Moles of glucose: n = 36 / 180 = 0.20 mol - Mass of solvent = 500 g = 0.500 kg - Mass of solution = 500 + 36 = 536 g - Volume of solution = 536 / 1.072 = 500 mL = 0.500 L
6
Calculation
(a) m = 0.20 / 0.500 = 0.40 mol/kg (b) M = 0.20 / 0.500 = 0.40 mol/L Note: The molar mass (180 g/mol) is exact by definition of the molecular formula and does not limit significant figures. The given masses (36 g, 500 g) and density (1.072 g/mL) are the measured values that govern precision.
7
Final answer
(a) Molality = 0.40 mol/kg (b) Molarity = 0.40 mol/L The numerical coincidence (both = 0.40) is specific to this problem's data. In general, molality ≠ molarity because their denominators differ.
8
Common trap
Using solution mass (536 g) instead of solvent mass (500 g) for molality. This gives m = 0.20/0.536 = 0.373, a wrong answer that appears plausible.
9
Similar NEET-style question
"4.5 g of urea (molar mass = 60 g/mol) is dissolved in 250 g of water. The solution density is 1.004 g/mL. Find the molality and molarity of the solution." (Answer: m = 0.30 mol/kg; M ≈ 0.295 mol/L) ---

Before solving, remember these

Definition

Concentration units

Mass percent (w/w) = (mass of solute/mass of solution)×100. Molarity (M) = moles of solute / L of solution. Molality (m) = moles of solute / kg of solvent. Mole fraction χ.

-- NCERT Class 12 Chemistry, Ch. 1, p. 4

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.4

Test yourself on this topic with real past-paper questions:

Practice this topic →

Free NEET study resources

Get a structured 30-day study plan and a complete formula booklet — delivered to your inbox instantly.