Osmotic Pressure

8 MCQs2 revision cards9-step worked example

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

Lesson

Osmotic pressure is the colligative property NEET uses to test whether you can handle unit conversions and the gas-equation-style formula π = CRT — and whether you forget to apply the Van't Hoff factor for electrolytes.

What is osmotic pressure? When a semipermeable membrane separates a solution from pure solvent, solvent molecules flow into the solution (osmosis). The minimum pressure that must be applied on the solution side to prevent this flow is the osmotic pressure, π (NCERT Class 12 Chemistry Chapter 1, page 22).

The formula:

π = CRT

where C is the molar concentration (mol/L) of the solute, R is the gas constant, and T is the absolute temperature in kelvin. For a known mass W of solute with molar mass M dissolved in V litres of solution, this becomes π = (W/M)(RT/V).

Why NEET favours this property: Osmotic pressure is measurable at room temperature, requires no heating/cooling, and is the preferred method for determining molar masses of biomolecules (proteins, polymers). The calculation is typically a two-step direct application — compute C from given mass data, then substitute into π = CRT.

The high-frequency trap: unit mismatch. The common distractor in NEET osmotic-pressure problems exploits unit confusion — mixing atmospheres with pascals, or litres with cubic metres. If R = 0.0821 L atm mol⁻¹ K⁻¹, then V must be in litres and π comes out in atm. If R = 8.314 J mol⁻¹ K⁻¹, then V must be in m³ and π comes out in Pa. Picking the wrong R-value combination gives you a distractor, not the answer.

For electrolytes: Multiply by the Van't Hoff factor i. If the solute is NaCl (i ≈ 2), the observed osmotic pressure is roughly double the value calculated assuming a non-electrolyte.

Watch-out: When a problem gives you mass in grams and volume in mL, convert both before substituting. NEET distractors are built from the answer you get when you skip one conversion.

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

Osmotic pressure of a solution depends on which of the following?

MCQ 2Easy RecallPractice

The formula for osmotic pressure of a dilute solution of a non-electrolyte is:

MCQ 3Easy RecallPractice

Osmotic pressure measurement is preferred over other colligative properties for determining the molar mass of proteins because:

MCQ 4Direct ApplicationPractice

6.0 g of urea (molar mass = 60 g/mol) is dissolved in 500 mL of solution at 300 K. The osmotic pressure is (R = 0.0821 L atm mol⁻¹ K⁻¹):

MCQ 5Direct ApplicationPractice

At 27 °C, the osmotic pressure of a solution containing 3.42 g of sucrose (molar mass = 342 g/mol) in 250 mL of solution is (R = 0.0821 L atm mol⁻¹ K⁻¹):

MCQ 6Direct ApplicationPractice

The osmotic pressure of a 0.10 M NaCl solution at 300 K is approximately (R = 0.0821 L atm mol⁻¹ K⁻¹, assume complete dissociation):

MCQ 7Concept TrapPractice

Two solutions — one of urea and one of NaCl — are prepared with the same molar concentration. Which will show higher osmotic pressure?

MCQ 8CalculationPractice

A solution contains 1.80 g of an unknown non-electrolyte dissolved in 200 mL of solution. At 27 °C, the osmotic pressure is 2.46 atm. The molar mass of the solute is (R = 0.0821 L atm mol⁻¹ K⁻¹):

Quick recall before you leave

Worked Example

Pattern: Determine molar mass from observed osmotic pressure (pattern NEET pattern: osmotic pressure problem).

1
Given
A solution is prepared by dissolving 2.50 g of an unknown non-electrolyte in 200.0 mL of solution. At 25 °C, the osmotic pressure is measured as 3.08 atm. R = 0.0821 L atm mol⁻¹ K⁻¹.
2
Required
Find the molar mass M of the solute.
3
Concept
For a dilute solution of a non-electrolyte, osmotic pressure is given by π = CRT, where C = (W/M)/V. Rearranging: M = WRT/(πV).
4
Formula
M = WRT / (πV)
5
Substitution
W = 2.50 g, R = 0.0821 L atm mol⁻¹ K⁻¹, T = 25 + 273 = 298 K, π = 3.08 atm, V = 200.0 mL = 0.2000 L. M = (2.50 × 0.0821 × 298) / (3.08 × 0.2000)
6
Calculation
Numerator: 2.50 × 0.0821 = 0.20525; 0.20525 × 298 = 61.165 Denominator: 3.08 × 0.2000 = 0.6160 M = 61.165 / 0.6160 = 99.3 g/mol Note on exact values: The temperature conversion (adding 273) and the volume conversion factor (÷1000) are exact operations. R = 0.0821 and π = 3.08 are given data — the answer is limited by the precision of these inputs (3 significant figures).
7
Final answer
M ≈ 99 g/mol (3 significant figures, consistent with the precision of the given data).
8
Common trap
The most common error: using V = 200 (in mL) directly instead of converting to 0.2000 L. This gives M = 61.165/616 = 0.0993, which is absurdly small and should immediately signal a unit mismatch. NEET distractors exploit exactly this kind of forgotten mL → L conversion.
9
Similar NEET-style question
A 1.20 g sample of a protein is dissolved in 100.0 mL of water. The osmotic pressure at 27 °C is 0.0821 atm. What is the molar mass of the protein? (R = 0.0821 L atm mol⁻¹ K⁻¹) *Approach:* M = WRT/(πV) = (1.20 × 0.0821 × 300)/(0.0821 × 0.1000) = 29.556/0.00821 ≈ 3600 g/mol — a realistic value for a small protein, confirming that osmotic pressure is the go-to method for biomolecule molar mass determination. ---

Before solving, remember these

Formula

Osmotic pressure

π = CRT = (n/V)RT = (W/M)·(RT/V), where C = molarity. Used for biomolecules (proteins) where small concentration produces measurable π.

-- NCERT Class 12 Chemistry, Ch. 1, p. 22

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.22

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

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Osmotic Pressure

Lesson

Practice MCQs

Quick recall before you leave

Osmotic pressure (non-electrolyte)

Osmotic pressure (electrolyte, with Van't Hoff factor)

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