JEE Mains Chemistry · Solutions

Osmosis and Osmotic Pressure

Solvent flows through a semipermeable membrane into the more concentrated solution, and the pressure that stops it is the osmotic pressure, π = iCRT.

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Why this matters

Nineteen PYQs, thirteen of them numeric, and six from 2026, more than any other page. Eleven use π = iCRT for a pressure or a molar mass, often of a protein or polymer; four match isotonic solutions by particle count; four ask which way the solvent flows and what can cross the membrane.

Concept 1 of 3: Osmotic pressure, π = iCRT

Osmotic pressure behaves like the pressure of an ideal gas made of the solute particles: moles per litre times

RT

. Because it is large even for tiny concentrations, it is the colligative property used to find the molar mass of a protein or a polymer at room temperature.

Definition

$\pi = iCRT$ , with $C$ in mol per litre of SOLUTION and $i$ the particles each formula unit gives (1 for a non-electrolyte, 2 for fully dissociated NaCl).
Molar mass: $M = \dfrac{wRT}{\pi V}$ ( $V$ in litres).
Units: $R = 0.083$ L bar/(K mol) $= 0.0821$ L atm/(K mol) $= 8.314$ kPa L/(K mol). $1$ bar $= 10^5$ Pa.
A column of solution: $\pi = h\rho g$ (in Pa with $h$ in m, $\rho$ in kg/m³).
Several solutes: add their particle concentrations.
Same solution at a new temperature: $\pi \propto T$ .
Mixing two solutions of the SAME concentration leaves the concentration, and so $\pi$ , unchanged.

Van 't Hoff equation for osmotic pressure

\pi = iCRT \qquad M = \frac{wRT}{\pi V} \qquad \pi = h\rho g

Worked example

1.0 g of a protein in 200 mL of aqueous solution has an osmotic pressure of 2.0 mbar at 290 K. Find its molar mass (

R = 0.083

L bar/(K mol)).

Practice this conceptself-check · 4 quick reps

The same idea in a real exam question:

JEE Mains · 2026 · 5 Apr 2026 Shift 2 · Q48Moderate

Example 1 · Solutions · Osmosis and Osmotic Pressure

20 g hemoglobin in a 1 L aqueous solution (A) at 300 K is separated from pure water by semi permeable membrane. At equilibrium the height of solution in a tube dipped in a solution (A) is found to be 80.0 mm higher than the tube dipped in water. The molar mass of hemoglobin is

\_\_\_\_

kg{mol}^{- 1}

.(Nearest integer) (Given :

g = 10{\text{ }ms}^{- 2},R = 8.3kPa{dm}^{3}{\text{ }K}^{- 1}{\text{ }mol}^{- 1}

, density of solution

= 1000\text{ }kg{\text{ }m}^{- 3}

)

Osmotic pressures do not add on mixing

Mixing two solutions of the same concentration gives the same concentration, so the same

\pi

. Adding the two pressures doubles the answer; that doubled value is an option.

Match R to the pressure unit

With

R = 0.083

L bar/(K mol) the pressure must be in bar; with

R = 8.314

it comes out in kPa (litres) or Pa (cubic metres). Put a pressure in Pa into the bar form and the molar mass is off by

10^5

Litres of solution, not of solvent

C

is moles per litre of SOLUTION. For a dilute solution 'in 200 mL of water' the two are taken as equal, but the formula itself wants the solution's volume.

Drill 10 more on osmotic pressure, π = icrt

Concept 2 of 3: Isotonic solutions, equal iC

At the same temperature, two solutions have the same osmotic pressure when they hold the same concentration of particles. So compare

iC

, not

C

: 0.1 M NaCl matches 0.2 M glucose, not 0.1 M glucose.

Definition

Isotonic: $i_1 C_1 = i_2 C_2$ at the same temperature.
Ions per formula unit (complete dissociation): NaCl, KCl 2; $\mathrm{CaCl_2}$ , $\mathrm{BaCl_2}$ , $\mathrm{K_2SO_4}$ , $\mathrm{Na_2SO_4}$ 3; $\mathrm{AlCl_3}$ 4; $\mathrm{K_4[Fe(CN)_6]}$ 5; $\mathrm{Al_2(SO_4)_3}$ 5.
Double salts: Mohr's salt $\mathrm{FeSO_4\cdot(NH_4)_2SO_4\cdot 6H_2O}$ gives 5 ions; carnallite $\mathrm{KCl\cdot MgCl_2\cdot 6H_2O}$ gives 5. Water of crystallisation adds no particles.
A solution isotonic with a cell or with blood has the same $\pi$ : $C = \dfrac{\pi}{iRT}$ , then grams per litre $= C \times M$ .
A partly dissociated salt: $i$ from the isotonic match, then $\alpha = \dfrac{i - 1}{n - 1}$ .

Isotonic condition

i_1 C_1 = i_2 C_2

Worked example

A 0.12 M solution of

\mathrm{K_2SO_4}

is fully dissociated. What molarity of glucose, and what molarity of

\mathrm{AlCl_3}

(fully dissociated), is isotonic with it?

Practice this conceptself-check · 4 quick reps

The same idea in a real exam question:

JEE Mains · 2026 · 21 Jan 2026 Shift 2 · Q47Moderate

Example 2 · Solutions · Osmosis and Osmotic Pressure

The osmotic pressure of a living cell in 12 atm at 300 K . The strength of sodium chloride solution that is isotonic with the living cell at this temperature is

\_\_\_\_

gL^{- 1}

. (Nearest integer) Given :

R = 0.08\text{ }L\text{ }atm{\text{ }K}^{- 1}{\text{ }mol}^{- 1}

Assume complete dissociation of NaCl (Given : Molar mass of Na and Cl are 23 and 35.5

g{mol}^{- 1}

respectively.)

Compare iC, not C

0.1 M NaCl and 0.1 M glucose are not isotonic: the salt gives twice the particles. Always multiply each concentration by its ion count before comparing.

Water of crystallisation is not a particle

\mathrm{KCl\cdot MgCl_2\cdot 6H_2O}

gives

\mathrm{K^+}

\mathrm{Mg^{2+}}

and three

\mathrm{Cl^-}

: 5 ions. The six water molecules join the solvent and add nothing.

Drill 3 more on isotonic solutions, equal ic

Concept 3 of 3: Direction of osmosis and reverse osmosis

A semipermeable membrane lets solvent through and holds solute back. The solvent moves to dilute the side with more particles. Push on that concentrated side with more than its osmotic pressure and the flow reverses.

Definition

Solvent flows from lower $iC$ (hypotonic) to higher $iC$ (hypertonic).
Ions and coloured species do not cross an ideal semipermeable membrane, so no reaction or colour appears on the other side.
As solvent leaves the dilute side, its molarity rises; the concentrated side is diluted and its molarity falls.
Reverse osmosis: pressure greater than $\pi$ on the CONCENTRATED side, through a true semipermeable membrane.

Situation	What happens	Why
Two solutions across a semipermeable membrane	Solvent flows from the side of lower iC to the side of higher iC	It dilutes the side with more particles
Ions on either side of the membrane	They stay on their own side; no precipitate or colour forms across it	The membrane passes solvent only 'Blue colour forms on both sides' is the planted false option.
Naming the sides	The side with higher iC is hypertonic, the other hypotonic	It has the higher osmotic pressure
Concentrations as osmosis runs	The concentrated side's molarity falls; the dilute side's rises	Water leaves the dilute side and enters the concentrated side
Reverse osmosis	Apply a pressure greater than π on the concentrated side	Pure solvent is pushed back to the dilute side, as in desalination
Membrane for reverse osmosis	Cellophane or parchment paper, not a porous partition	A porous partition lets the solute through as well

The membrane decides what moves (solvent only); the particle count decides which way.

Practice this conceptself-check · 4 quick reps

The same idea in a real exam question:

JEE Mains · 2026 · 6 Apr 2026 Shift 2 · Q30Moderate

Example 3 · Solutions · Osmosis and Osmotic Pressure

Given below are two statements :

Chamber 1	Semipermeable membrane	Chamber 2
18 g glucose in 100 mL aqueous solution		30 g glucose in 250 mL aqueous solution

Statement I:

H_{2}O

molecules move from the chamber 1 to chamber 2. Statement II: The osmotic pressure of a solution prepared by dissolving 50 mg of potassium sulphate (molar mass

= 174\text{ }g/mol

) in 2 L of water (at

27^{\circ}C

) is 0.0107 bar. (Given :

R = 0.083{dm}^{3}

bar

K^{-1}{\text{ }mol}^{-1}

and assume complete dissociation of electrolyte) In the light of the above statements, choose the correct answer from the options given below :

Solvent flows towards the concentrated side

Osmosis moves solvent from the hypotonic to the hypertonic solution, never the other way. A statement that osmosis runs from hypertonic to hypotonic is false.

Reverse osmosis pushes on the concentrated side

The applied pressure must exceed

\pi

and act on the concentrated solution. Pressure on the dilute side only speeds up ordinary osmosis.

Drill 3 more on direction of osmosis and reverse osmosis

Summary — formulas & gotchas at a glance

A revision cheat-sheet for the formulas and gotchas above. Click any concept name to jump back to its full explanation.

Formulas (2)

Osmotic pressure, π = iCRT
Van 't Hoff equation for osmotic pressure
$\pi = iCRT \qquad M = \frac{wRT}{\pi V} \qquad \pi = h\rho g$
Isotonic solutions, equal iC
Isotonic condition
$i_1 C_1 = i_2 C_2$

Reference tables (1)

Direction of osmosis and reverse osmosis6 rows

Situation	What happens	Why
Two solutions across a semipermeable membrane	Solvent flows from the side of lower iC to the side of higher iC	It dilutes the side with more particles
Ions on either side of the membrane	They stay on their own side; no precipitate or colour forms across it	The membrane passes solvent only 'Blue colour forms on both sides' is the planted false option.
Naming the sides	The side with higher iC is hypertonic, the other hypotonic	It has the higher osmotic pressure
Concentrations as osmosis runs	The concentrated side's molarity falls; the dilute side's rises	Water leaves the dilute side and enters the concentrated side
Reverse osmosis	Apply a pressure greater than π on the concentrated side	Pure solvent is pushed back to the dilute side, as in desalination
Membrane for reverse osmosis	Cellophane or parchment paper, not a porous partition	A porous partition lets the solute through as well

The membrane decides what moves (solvent only); the particle count decides which way.

Watch out for (7)

Osmotic pressures do not add on mixing
→ Osmotic pressure, π = iCRT
Match R to the pressure unit
→ Osmotic pressure, π = iCRT
Litres of solution, not of solvent
→ Osmotic pressure, π = iCRT
Compare iC, not C
→ Isotonic solutions, equal iC
Water of crystallisation is not a particle
→ Isotonic solutions, equal iC
Solvent flows towards the concentrated side
→ Direction of osmosis and reverse osmosis
Reverse osmosis pushes on the concentrated side
→ Direction of osmosis and reverse osmosis

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