Osmotic pressure is the pressure that results when a solution containing dissolved particles or molecules exerts itself on a semipermeable membrane. It’s an example of colligative properties: properties that depend only on the number of particles in solution rather than their identity. The amount of osmotic pressure depends on the size and concentration of particles in the solution, as well as on how many there are compared to what else might be dissolved in it (e.g., sugar). Osmotic pressure can be measured using either a manometer or an osmometer.

What is osmotic pressure?

Osmotic pressure is the pressure that a solution exerts on a semipermeable membrane in order to balance the amount of solvent and solute flowing across it. It is one of several methods by which cells regulate their internal environment, including changes in volume, pH, and temperature. Osmotic pressure is caused by dissolved particles—the most common being salts such as sodium chloride (NaCl) or sugar molecules like glucose—that are separated from the solution by a semi-permeable membrane. This means that some substances can pass through this barrier but others cannot.

How to calculate osmotic pressure

In order to determine the osmotic pressure of a solution, you’ll need to know a few things:

  • The number of ions
  • The osmotic coefficient (keep reading for more about this)
  • The concentration
  • The temperature

Coefficients for osmotic pressure formula

The osmotic pressure formula is presented in a few different forms, but they all contain the same basic variables.

The first thing you need to know about each form of this equation is that molar mass (M) refers to the mass of one mole of something; it’s just shorthand for 1 mole. The molar masses of solvents are always given in grams per mole, while those for solutes are listed in both grams per mole and kilograms per kilogram (kg/kg). The number of moles (n) represents how many molecules there are within your solution or mixture—if you have one gram or kilogram of water and add 0.1 grams or kilograms by weight of salt, then n = 1 because there is now one gram or kilogram total present.

If you’re using an online calculator to solve osmotic pressure problems, it may ask how many moles were added instead; if so make sure that this information matches what’s written on your problem sheet!


What is osmotic pressure?

Osmotic pressure is defined as the pressure that must be applied to the solution side to stop fluid movement when a semipermeable membrane separates a solution from pure water.

What determines osmotic pressure?

Osmotic pressure is affected by concentration and temperature.

Does high osmolarity mean more water?

A solution with high osmolarity has fewer water molecules with respect to solute particles.