# What is osmotic pressure, and what is it dependent on?

##### 1 Answer
Jul 4, 2017

Osmotic pressure is the pressure that needs to be applied to one side of a semi-permeable membrane to prevent solvent flow across the membrane into the more concentrated side, and is given by:

$\Pi = i M R T$,

where:

• $i$ is the van't Hoff factor for the solute(s), i.e. the effective number of particles in solution per formula unit.
• $M$ is the solution concentration in... well, $\text{mol/L}$, molarity, $\text{M}$.
• $\Pi$ is the osmotic pressure in $\text{atm}$, if $R$ is the universal gas constant in $\text{L"cdot"mol/atm"cdot "K}$.
• $T$ is temperature in $\text{K}$, as usual.

Thus, osmotic pressure is dependent on

• the electrolytic features of the solute (the stronger the electrolyte, the more particles per formula unit, and thus the higher the $\Pi$ needed to stop a larger number of solute particles from sucking up the solvent)
• the concentration of the solution on one side of the membrane (the higher it is, the greater the $\Pi$ needed to stop a higher concentration of solute from sucking up the solvent)
• the temperature of the solution (the higher it is, the greater $\Pi$ is to stop the faster solvent flow in warmer environments)

These are all direct proportionalities, i.e. as $T \uparrow$, $\Pi \uparrow$, etc.