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. 22Osmotic Pressure
8 MCQs6 revision cards9-step worked example
Source: NCERT SolutionsPYQ coverage: NEET 2021, 2024Official key: NTA-verifiedLast updated: 27 Sep 2026
Osmotic Pressure, explained for NEET
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.
Can you answer these Osmotic Pressure 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?
Show answer and why every option is right or wrong
Answer: D. Osmotic pressure is a colligative property — it depends on the number of solute particles per unit volume (molar concentration), not on the nature of solute, solvent, or membrane (NCERT Class 12 Chemistry Chapter 1, page 22).
Why A is wrong: A is wrong because osmotic pressure is a colligative property — it depends on the number of solute particles, not the chemical nature of the solute.
Why B is wrong: B is wrong because while the solvent determines the medium, osmotic pressure depends on solute particle concentration, not solvent identity.
Why C is wrong: C is wrong because the membrane only enables the phenomenon by being selectively permeable; the magnitude of π depends on solute concentration.
MCQ 2Easy RecallPractice
The formula for osmotic pressure of a dilute solution of a non-electrolyte is:
Show answer and why every option is right or wrong
Answer: A. The van't Hoff equation for osmotic pressure is π = CRT, analogous in form to the ideal gas equation PV = nRT, where C = n/V is the molar concentration (NCERT Class 12 Chemistry Chapter 1, page 22).
Why B is wrong: B is wrong because dividing C by RT inverts the relationship — osmotic pressure increases with both concentration and temperature, so they must be multiplied.
Why C is wrong: C is wrong because this places concentration in the denominator, which would mean π decreases as concentration increases — the opposite of colligative behaviour.
Why D is wrong: D is wrong because there is no squared dependence on concentration in the osmotic pressure equation for dilute solutions.
MCQ 3Easy RecallPractice
Osmotic pressure measurement is preferred over other colligative properties for determining the molar mass of proteins because:
Show answer and why every option is right or wrong
Answer: D. Proteins have high molar masses, so their molal concentrations are very low — boiling-point elevation and freezing-point depression become too small to measure accurately. Osmotic pressure, however, gives measurable values even at low concentrations and at room temperature, avoiding protein denaturation (NCERT Class 12 Chemistry Chapter 1, page 22).
Why A is wrong: A is wrong because many proteins are soluble in aqueous media — that is precisely why their solutions exhibit osmotic pressure.
Why B is wrong: B is wrong because osmotic pressure is measured at room temperature — no boiling is required, which is one of its advantages.
Why C is wrong: C is wrong because protein solutions do exert osmotic pressure; this is in fact the basis for the measurement.
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):
Show answer and why every option is right or wrong
Answer: B. Moles of urea = 6.0/60 = 0.10 mol. V = 500 mL = 0.500 L. C = 0.10/0.500 = 0.200 mol/L. π = CRT = 0.200 × 0.0821 × 300 = 4.926 ≈ 4.93 atm (NCERT Class 12 Chemistry Chapter 1, page 22).
Why A is wrong: A is wrong — this is the result if you use V = 1.0 L instead of 0.500 L, halving the concentration and therefore the osmotic pressure (trap: unit conversion error on volume).
Why C is wrong: C is wrong — this is the value if you mistakenly use V = 5.0 L (reading 500 mL as 5000 mL), giving a tenfold dilution error (trap: mL-to-L conversion).
Why D is wrong: D is wrong: 24.6 atm corresponds to C = 1.0 mol/L, five times the actual 0.200 mol/L (0.10 mol in 0.500 L).
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):
Show answer and why every option is right or wrong
Answer: C. Moles = 3.42/342 = 0.0100 mol. V = 250 mL = 0.250 L. C = 0.0100/0.250 = 0.0400 mol/L. T = 27 + 273 = 300 K. π = 0.0400 × 0.0821 × 300 = 0.9852 ≈ 0.985 atm (NCERT Class 12 Chemistry Chapter 1, page 22).
Why A is wrong: A is wrong — this arises if you use V = 1.0 L instead of 0.250 L, effectively diluting the concentration fourfold.
Why B is wrong: B is wrong — this value results from using V = 0.500 L instead of 0.250 L, i.e. misreading 250 mL as 500 mL (trap: volume conversion error).
Why D is wrong: D is wrong — this would arise from using V = 0.0250 L (misplacing the decimal by one factor of 10 when converting mL to L), making the concentration 10× too high.
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):
Show answer and why every option is right or wrong
Answer: C. NaCl → Na⁺ + Cl⁻, so i = 2. π = iCRT = 2 × 0.10 × 0.0821 × 300 = 4.926 ≈ 4.93 atm (NCERT Class 12 Chemistry Chapter 1, page 22).
Why A is wrong: A is wrong — this is the value without the Van't Hoff factor (π = CRT = 0.10 × 0.0821 × 300 = 2.46 atm). NaCl is an electrolyte; you must multiply by i = 2 (trap: forgetting the Van't Hoff factor for ionic solutes).
Why B is wrong: B is wrong — this corresponds to i = 0.5, as if you divided by 2 instead of multiplying, which has no physical basis.
Why D is wrong: D is wrong — this would require i = 3 (e.g. CaCl₂ → Ca²⁺ + 2Cl⁻). NaCl gives only 2 ions per formula unit, so i = 2, not 3.
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?
Show answer and why every option is right or wrong
Answer: B. Osmotic pressure depends on the total number of solute particles. NaCl dissociates into Na⁺ and Cl⁻ (i ≈ 2), effectively doubling the particle concentration compared to urea (a non-electrolyte, i = 1). At the same molarity, π(NaCl) ≈ 2 × π(urea) (NCERT Class 12 Chemistry Chapter 1, page 22).
Why A is wrong: A is wrong — urea is a non-electrolyte (i = 1) and does not dissociate; it contributes fewer particles per mole than NaCl.
Why C is wrong: C is wrong — colligative properties depend on the number of particles, not just molarity. NaCl dissociates, so it has more particles at the same molarity.
Why D is wrong: D is wrong — temperature affects both solutions equally (both multiplied by the same RT), so the ratio of their osmotic pressures is determined by i, not T.
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):
Show answer and why every option is right or wrong
Answer: A. Step 1: π = (W/M)(RT/V). Rearranging: M = WRT/(πV). Step 2: T = 300 K, V = 0.200 L. M = (1.80 × 0.0821 × 300)/(2.46 × 0.200) = 44.334/0.492 = 90.1 ≈ 90 g/mol (NCERT Class 12 Chemistry Chapter 1, page 22).
Why B is wrong: B is wrong — this is the result if you use V = 0.400 L (misreading 200 mL as 400 mL), which doubles the denominator and halves the molar mass (trap: volume conversion error).
Why C is wrong: C is wrong — this arises if you use V = 0.100 L instead of 0.200 L, halving the denominator and doubling M.
Why D is wrong: D is wrong — this results from using V = 0.300 L, giving M = 44.334/0.738 ≈ 60, a plausible-looking distractor from an incorrect volume substitution.
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Osmotic Pressure: quick recall before you leave
How do you solve a Osmotic Pressure question? A 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.
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What to remember before solving Osmotic Pressure questions
Which Osmotic Pressure formulas do you need for NEET?
Osmotic pressure
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
More in Solutions: 4 exam traps and mistakes · 7 formulas · 2 question patterns from its other lessons.
Osmotic Pressure questions from past NEET papers
2 questions from NEET 2021, 2024. Answers verified against NTA official keys.
NEET 2024Revised key
137°C
2310°C
325.73°C
412.05°C
NTA Answer: Option 1(revised_final)
NEET 2021
1P₃ > P₁ > P₂
2P₂ > P₁ > P₃
3P₁ > P₂ > P₃
4P₂ > P₃ > P₁
NTA Answer: Option 2(final)
How does NEET ask about Osmotic Pressure?
Recurring question shapes from past papers. Each pattern shows why wrong options look tempting.
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
Sources
NCERT refs: Class 12 Chemistry Chapter 1, p.22
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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