Q.A solution concentration is expressed in molarity and not in molality while considering osmotic pressure. Why?
Concept understanding — Osmotic Pressure
Osmotic Pressure: The Push of Pure Solvent
Imagine you have a U-shaped tube with a special membrane at the bottom that only lets water molecules pass through — not sugar molecules. On one side you put pure water, on the other side you put a sugar solution. What happens?
Water moves from the pure side into the solution side. The solution level rises. This is osmosis — the spontaneous net movement of solvent across a semipermeable membrane from a region of lower solute concentration to higher solute concentration.
But here's the key question: what if you don't want that level to rise? What if you want to keep the solution side exactly where it is?
You would have to push down on the solution side with extra pressure — just enough to stop the water from coming in. That extra pressure is osmotic pressure.
Osmotic pressure is not a pressure the solution "has" inside it. It is the external pressure you must apply to prevent osmosis. Think of it as the "resistance pressure" that exactly balances the tendency of solvent to dilute the solution.
The Precise Definition
Osmotic pressure (Π) is the minimum excess pressure that must be applied to a solution to prevent the inward flow of solvent across a semipermeable membrane.
The membrane must be permeable only to solvent molecules, not to solute particles. This is the defining condition — if the membrane leaks solute, you don't get true osmotic pressure.
The van't Hoff Equation
For dilute solutions, osmotic pressure follows a beautifully simple law:
Π=CRT
Where:
- Π = osmotic pressure (in atm or Pa)
- C = molar concentration of solute (mol/L or mol/m³)
- R = universal gas constant
- T = absolute temperature (K)
This is van't Hoff's law of osmotic pressure. It looks exactly like the ideal gas law (PV=nRT rearranged as P=(n/V)RT), and that's no coincidence — van't Hoff noticed that solute particles in dilute solution behave like gas molecules bouncing around, creating a "pressure" against the membrane.
This equation works only for non-electrolyte solutions at low concentrations. For electrolytes, you must include the van't Hoff factor i: Π=iCRT. A 0.1 M NaCl solution gives nearly twice the osmotic pressure of a 0.1 M glucose solution because NaCl dissociates into two ions.
Why It Matters for Macromolecules
Here's where osmotic pressure becomes a powerful tool. Suppose you have a protein — say, hemoglobin — and you want to know its molar mass. You can't easily measure its concentration in mol/L because you don't know the molar mass yet. But you can measure:
- The mass of protein dissolved (say, w grams in V liters)
- The osmotic pressure Π of that solution
Since C=n/V=(w/M)/V, where M is the molar mass:
Π=MVwRT
Rearrange:
M=ΠVwRT
| Property | Why osmotic pressure wins |
|----------|---------------------------|
| Boiling point elevation | Very small for macromolecules — hard to measure |
| Freezing point depression | Very small — same problem |
| Osmotic pressure | Large enough to measure accurately even for dilute solutions |
A 1% protein solution might give a freezing point depression of only 0.001°C — nearly impossible to measure precisely. But the same solution gives an osmotic pressure of several cm of water — easily measurable with a simple manometer.
The Intuition in One Sentence
Osmotic pressure is the "push" you need to apply to stop water from rushing in to dilute a solution — and because that push depends only on the number of solute particles (not their size or identity), it tells you exactly how many particles you have, which gives you the molar mass.
| Quantity | Symbol | Typical unit |
|----------|--------|--------------|
| Osmotic pressure | Π | atm or Pa |
| Concentration | C | mol/L |
| Gas constant | R | 0.0821 L·atm/mol·K |
| Temperature | T | K |
For exam problems: always check whether the solute is an electrolyte, convert temperature to Kelvin, and ensure units of R match your pressure units.
Because it involves a direct formula application, Osmotic Pressure is a recurring numerical topic in CBSE Class 12 board papers as well as JEE Main and NEET — students commonly search for "Osmotic Pressure formula and derivation" or "Osmotic Pressure class 12 chemistry solved examples", and this concept sits squarely within the NCERT-aligned Solutions unit.
Osmotic pressure measurements are made at one fixed, constant temperature, so there's no need for the temperature-independent unit molality -- molarity works fine.
Because osmotic pressure is measured at a single, constant temperature, molarity (which changes with temperature via volume) causes no problem here -- unlike boiling-point/freezing-point work, where temperature itself varies and molality's temperature-independence is needed.
Step 1. Molarity is defined per unit VOLUME of solution, and liquid volumes change slightly with temperature -- so molarity itself is technically temperature-dependent.
Step 2. In boiling-point-elevation and freezing-point-depression measurements, the whole point is to compare behaviour AT DIFFERENT temperatures, so a temperature-INdependent concentration unit (molality, defined per unit mass) is needed there.
Step 3. Osmotic pressure, by contrast, is always measured at ONE single, fixed temperature -- there's no need to compare across different temperatures within a single measurement, so molarity's mild temperature-dependence causes no practical issue, and it is used because it directly matches pi = MRT's derivation from n2/V.
Osmotic pressure measurements are made at one constant temperature, so molarity works perfectly well there; molality's temperature-independence is only needed when temperature itself is varying, as in boiling-point/freezing-point work.
Contrast the fixed-temperature nature of osmotic pressure measurement with the varying-temperature nature of boiling/freezing-point measurement.
- Reversing the logic and claiming molality is used for osmotic pressure -- the chapter is explicit that molarity, not molality, is the unit used in pi=MRT.
Showing the 12 most recent of 36 on this concept.
- CBSE 2026Set DZ1 markMCQQ.Which formula is used to calculate osmotic pressure?(a) π=CRT(b) π=VRT(c) π=PVT(d) π=nRT
›Reveal solutionSolution
Osmotic pressure is a colligative property given by π=CRT, so option (a) is correct.
Osmotic pressure (π) is the extra pressure that must be applied on the solution side of a semipermeable membrane to just stop the net flow of solvent into the solution. For a dilute solution it depends only on the number of solute particles per unit volume, i.e. the molar concentration C=n/V.
The van't Hoff equation for osmotic pressure is:
π=CRT=VnRT
where C = molar concentration (mol L−1), R = gas constant, T = absolute temperature. This resembles the ideal-gas equation because both depend on the concentration of particles.
Checking the options: π=VRT, π=PVT and π=nRT are dimensionally/physically wrong (osmotic pressure cannot equal nRT without dividing by volume).
✓Final answer(a) π=CRT.
- CBSE 2026Set ANNUAL1 markMCQQ.Isotonic solutions have the same:(a) Normality(b) Molarity(c) Osmotic pressure(d) Formality
›Reveal solutionSolution
Isotonic solutions have equal osmotic pressure, which is why cells neither swell nor shrink when placed in an isotonic medium.
Osmotic pressure (π) of a solution is given by π = CRT, where C is the molar concentration of solute particles. Two solutions are said to be isotonic with respect to each other when they exhibit the SAME osmotic pressure at the same temperature. When such solutions are separated by a semipermeable membrane, there is no net flow of solvent from one to the other — this is why isotonic saline (0.9% NaCl) is used for intravenous drips, since it matches the osmotic pressure of blood plasma and doesn't cause red blood cells to swell (hypotonic) or shrink (hypertonic).
Normality and Molarity concern the concentration of solute in terms of equivalents/moles per litre and do not by themselves guarantee equal osmotic pressure (different solutes at the same molarity can dissociate to different extents, e.g. NaCl vs glucose), and "formality" is simply an alternative way of expressing concentration of ionic solids, similarly not the defining criterion for isotonicity.
✓Final answer(c) Osmotic pressure — by definition, isotonic solutions have equal osmotic pressure.
- CBSE 2026Set ANNUAL1 markMCQQ.A plant cell shrinks when it is kept in a(a) Hypotonic solution(b) Hypertonic solution(c) Isotonic solution(d) Pure water
›Reveal solutionSolution
A plant cell placed in a hypertonic solution loses water to its surroundings by osmosis and shrinks; this is called plasmolysis.
A solution is hypertonic relative to the cell sap if it has a higher solute concentration (higher osmotic pressure) than the cell's interior. Water always moves from a region of lower solute concentration (higher water potential) to higher solute concentration (lower water potential) across the semi-permeable cell membrane.
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In a hypotonic solution, water enters the cell, making it swell (and possibly burst in an animal cell, though a plant cell becomes turgid due to its wall).
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In an isotonic solution, there is no net movement of water.
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In a hypertonic solution, water leaves the cell, causing the protoplasm to shrink away from the cell wall - this is plasmolysis.
✓Final answer(b) Hypertonic solution.
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- CBSE 2026Set ANNUAL1 markMCQQ.Sea water is converted into fresh water based upon the phenomenon of:(a) Diffusion(b) Osmosis(c) Plasmolysis(d) Reverse Osmosis
›Reveal solutionSolution
Desalination of sea water to get fresh water uses reverse osmosis.
In normal osmosis, solvent flows from a dilute solution to a concentrated solution through a semipermeable membrane. If a pressure greater than the osmotic pressure of the sea water is applied on the sea-water side, the flow of solvent is reversed — pure water is forced from the concentrated (saline) side through the membrane to the dilute side, leaving the salts behind. This process, reverse osmosis, is the principle used in desalination plants to convert sea water into fresh, potable water.
✓Final answer(d) Reverse Osmosis.
- CBSE 2026Set ANNUAL1 markMCQQ.The pressure of solution that just prevents the flow of solvent is called:(a) Vapour Pressure(b) Osmotic Pressure(c) Partial Pressure(d) Gas pressure
›Reveal solutionSolution
The pressure that just stops solvent flow (osmosis) is the osmotic pressure.
When a solution is separated from pure solvent by a semipermeable membrane, solvent flows into the solution (osmosis). The excess pressure that must be applied on the solution side to just prevent this net inflow of solvent is called the osmotic pressure. It is a colligative property, given by pi = CRT (van't Hoff equation).
✓Final answer(b) Osmotic Pressure.
- CBSE 2025Set D1 markMCQQ.Isotonic solutions have equal(a) osmotic pressure(b) vapour pressure(c) relative lowering of vapour pressure(d) elevation of boiling point
›Reveal solutionSolution
Isotonic solutions have equal osmotic pressure.
By definition, two solutions are isotonic if they exert the same osmotic pressure at the same temperature. Because osmotic pressure pi = C R T, isotonic solutions must have the same molar concentration of particles.
If two solutions are isotonic, there is no net movement of solvent across a semipermeable membrane between them.
✓Final answer(a) osmotic pressure.
- CBSE 2025Set ANNUAL1 markMCQQ.The porous membrane used in reverse osmosis plant is made up by:(a) Cellulose acetone(b) Potassium nitrate(c) Mercuric iodide(d) Starch
›Reveal solutionSolution
The semi-permeable membrane used in reverse osmosis water purification plants is made of cellulose acetate.
In reverse osmosis, a pressure greater than the osmotic pressure of the solution (e.g. seawater) is applied on the solution side, forcing pure solvent (water) to flow through a semi-permeable membrane from the concentrated to the dilute side — the reverse of normal osmosis. The membrane commonly used for this purpose in desalination/purification plants is made of cellulose acetate — a synthetic semi-permeable membrane that allows water molecules to pass through but rejects dissolved salts. (Option (a) in this paper prints it as "cellulose acetone", which is a typographical slip for cellulose acetate.)
✓Final answer(a) Cellulose acetate
- CBSE 2025Set ANNUAL1 markMCQQ.Which of the following colligative properties is used to determine molar masses of proteins, polymers or other macromolecules?(a) Depression in freezing point(b) Relative lowering of vapour pressure(c) Osmotic pressure(d) Elevation in boiling point
›Reveal solutionSolution
Colligative properties are inversely proportional to molar mass, so for very large molecules (proteins, polymers) most of them become too small to measure precisely — except osmotic pressure, which stays measurable even in very dilute solutions.
Why the other options fail
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(a) Depression in freezing point ΔTf=Kfm and (d) Elevation in boiling point ΔTb=Kbm: for a macromolecule of large M, the molality m=M×w1w2×1000 is tiny, so ΔTf/ΔTb become too small (often a few thousandths of a kelvin) to measure accurately with an ordinary thermometer.
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(b) Relative lowering of vapour pressure p1∘p1∘−p1=x2 also becomes vanishingly small and is experimentally hard to measure with the needed precision for dilute macromolecular solutions.
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(c) Osmotic pressure π=Vn2RT=CRT is directly proportional to molar concentration but is a comparatively LARGE, easily-measurable quantity (of the order of atmospheres) even for very dilute solutions, because of the factor RT. This makes it the preferred method experimentally.
✓Final answer(c) Osmotic pressure — because it remains large enough to measure accurately even in the very dilute solutions typical of proteins and polymers.
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- CBSE 2025Set ANNUAL1 markQ.What are hypertonic solutions?
›Reveal solutionSolution
A hypertonic solution has a greater osmotic pressure (higher effective solute concentration) than a reference solution, so it draws water out of a cell placed in it.
Definition
When two solutions are compared, the one with the higher osmotic pressure (i.e. higher concentration of osmotically active solute particles) is called the hypertonic solution relative to the other (which is then called hypotonic).
If a biological cell (or any solution enclosed by a semi-permeable membrane) is placed in a hypertonic solution, the solvent (water) flows out of the cell (from the region of lower solute concentration/osmotic pressure inside the cell to the higher one outside), through osmosis, causing the cell to shrink — called plasmolysis in plant cells or crenation in animal cells (e.g. red blood cells shrivel in a hypertonic saline solution).
✓Final answerA hypertonic solution has a higher osmotic pressure than the solution it is compared with; a cell placed in it loses water and shrinks.
- CBSE 2024Set D1 markMCQQ.Isotonic solutions have the same(a) Density(b) Normality(c) Strength(d) Molar concentration
›Reveal solutionSolution
Isotonic solutions have the same osmotic pressure. Since osmotic pressure pi = CRT, equal pi at the same temperature means equal molar concentration C.
Osmotic pressure of a dilute solution: pi = CRT, where C is molar concentration, R the gas constant and T the temperature.
Two solutions are isotonic when they have equal osmotic pressures. At the same temperature this requires the same molar concentration of solute particles. (They need not have the same density, normality or mass strength.)
✓Final answer(D) Molar concentration.
- CBSE 2024Set D1 markMCQQ.The osmotic pressure of a solution is represented by which of the following equations?(a) pi = CR/T(b) pi/C = RT(c) pi = CT/R(d) pi = RT/C
›Reveal solutionSolution
Osmotic pressure: pi = CRT, i.e. pi/C = RT.
For a dilute solution the osmotic pressure (pi) is given by the van't Hoff equation:
pi = CRT
where C is the molar concentration (n/V), R is the gas constant and T is the absolute temperature. Rearranging gives pi/C = RT, which matches option (b). The other options do not reproduce pi = CRT.
✓Final answer(b) pi/C = RT — the rearranged form of pi = CRT.
- CBSE 2024Set ANNUAL1 markQ.The two solutions with equal osmotic pressure are called ______.
›Reveal solutionSolution
Two solutions having equal osmotic pressure at a given temperature are called isotonic solutions.
Osmotic pressure (pi) is the colligative property that measures the pressure needed to stop the net flow of solvent across a semipermeable membrane. If two solutions have exactly the same osmotic pressure at the same temperature, no net osmosis occurs when they are separated by a semipermeable membrane and placed in contact - such solutions are called isotonic solutions.
If one solution has a higher osmotic pressure than the other, it is called hypertonic with respect to the other, and the one with the lower osmotic pressure is called hypotonic. This concept is important biologically - e.g. a 0.9% NaCl (normal saline) solution is isotonic with human blood plasma, which is why it is used in intravenous drips instead of pure water (which would be hypotonic and cause blood cells to swell and burst).
✓Final answerIsotonic solutions.
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