Chemistry · Solutions · NEET
Osmosis is when solvent molecules (usually water) move on their own through a semipermeable membrane. They flow from the dilute side (less solute) to the concentrated side (more solute). Only the small solvent molecules can pass; the solute stays behind. This makes both sides move toward equal concentration. Example: a raisin swells in water because water enters it by osmosis.
Osmosis is the flow of solvent. Osmotic pressure (π) is the exact external pressure you must apply on the more concentrated solution to STOP that flow. So osmosis is the process; osmotic pressure is the force that just balances it. A higher solute concentration means water wants to enter more strongly, so the osmotic pressure is higher.
The formula is π = CRT. Here C is the molar concentration (mol/L, sometimes written n/V), R is the gas constant (0.0821 L atm mol⁻¹ K⁻¹ or 0.083 L bar mol⁻¹ K⁻¹), and T is temperature in Kelvin. If R is in L atm, π comes out in atm; if R is in L bar, π comes out in bar. Always convert temperature to Kelvin (K = °C + 273).
A colligative property depends only on the NUMBER of solute particles, not on what they are. In π = CRT, only concentration (number of moles per litre) matters. So 1 mole of glucose and 1 mole of urea in the same volume give the same osmotic pressure, even though they are different substances. This is why NEET loves it.
A semipermeable membrane is a thin barrier with tiny pores. It lets small solvent molecules (water) pass through but blocks the larger solute particles (like sugar or salt). Common examples are a pig's bladder, cell membranes, and copper ferrocyanide films. Without a semipermeable membrane, osmosis cannot happen — you get plain diffusion instead.
In diffusion, ALL particles (both solute and solvent) spread out to fill space, and no membrane is needed. In osmosis, ONLY the solvent moves, and it must pass through a semipermeable membrane. So osmosis is a special, membrane-controlled, solvent-only kind of movement.
If you dissolve the same mass of two solutes, the one with the smaller molar mass gives MORE moles (moles = mass ÷ molar mass). More moles means higher concentration C, and since π = CRT, higher C means higher osmotic pressure. So smaller molar mass → higher osmotic pressure. This is the exact idea tested in NEET 2021.
The following solutions were prepared by dissolving 10 g glucose (C6H12O6) in 250 mL of water (P1), 10 g of urea (CH4N2O) in 250 mL of water (P2) and 10 g of sucrose (C12H22O11) in 250 mL of water (P3). The right option for the decreasing order of osmotic pressure of these solutions is:
The plot of osmotic pressure (π) 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)
Try the real previous-year questions from this chapter — each with the answer and a full solution.
π = CRT, where C is molar concentration (mol/L), R is the gas constant, and T is temperature in Kelvin. It can also be written π = nRT/V or π = wRT/(MV).
Yes. It depends only on the number of solute particles in solution, not on their nature. This is why it is one of the four colligative properties in NEET.
Atmospheres (atm) or bar. Use R = 0.0821 L atm mol⁻¹ K⁻¹ for atm, or R = 0.083 L bar mol⁻¹ K⁻¹ for bar. Match your R to the unit asked.
The solute with the smallest molar mass, because it gives the most moles and therefore the highest concentration. For example, urea gives a higher π than glucose or sucrose for the same mass and volume.
Because osmotic pressure values are large and easy to measure at room temperature, even for very dilute solutions and large molecules like proteins. This gives more accurate molar masses than boiling point or freezing point methods.
