Osmosis is the movement of solvent, usually water, across a semipermeable membrane from a dilute solution toward a more concentrated solution. It matters because it helps explain how cells gain or lose water, how plant roots absorb water, and how medical IV fluids are designed. The membrane lets some particles pass through while blocking others, so concentration differences can produce a real physical effect.
Osmosis continues until the driving tendency for water movement is balanced by pressure or concentration becomes effectively equal.
Key Facts
- Osmosis is solvent movement through a semipermeable membrane from lower solute concentration to higher solute concentration.
- Osmotic pressure is the pressure required to stop osmosis.
- For dilute solutions, osmotic pressure is given by π = iMRT.
- In π = iMRT, i is the van't Hoff factor, M is molarity, R = 0.0821 L atm mol^-1 K^-1, and T is temperature in kelvin.
- Isotonic solutions have equal effective solute concentrations, so there is no net water movement.
- A hypertonic solution has higher solute concentration than the comparison solution, while a hypotonic solution has lower solute concentration.
Vocabulary
- Osmosis
- Osmosis is the net movement of solvent across a semipermeable membrane toward the side with higher solute concentration.
- Semipermeable membrane
- A semipermeable membrane allows certain particles, such as water molecules, to pass while blocking others, such as many dissolved solutes.
- Osmotic pressure
- Osmotic pressure is the external pressure needed to prevent net solvent flow into a solution through a semipermeable membrane.
- Tonicity
- Tonicity describes how a solution affects the water balance of a cell or another solution separated by a membrane.
- Van't Hoff factor
- The van't Hoff factor is the number of dissolved particles produced per formula unit of a solute in solution.
Common Mistakes to Avoid
- Saying water moves from high solute concentration to low solute concentration is wrong because osmosis describes water moving toward the higher solute concentration side.
- Ignoring the semipermeable membrane is wrong because osmosis only occurs when the membrane allows solvent through but restricts at least some solute.
- Using Celsius in π = iMRT is wrong because gas-law-style equations require temperature in kelvin.
- Treating all solutes as i = 1 is wrong because ionic compounds can dissociate into multiple particles and increase osmotic pressure.
Practice Questions
- 1 A 0.200 M glucose solution is separated from pure water by a semipermeable membrane at 298 K. Using i = 1 and R = 0.0821 L atm mol^-1 K^-1, calculate the osmotic pressure.
- 2 A 0.150 M NaCl solution is at 310 K. Assume complete dissociation so i = 2. Calculate π using R = 0.0821 L atm mol^-1 K^-1.
- 3 A red blood cell is placed in a solution that is hypertonic compared with its cytoplasm. Predict the direction of net water movement and describe what happens to the cell.