Question

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

Answer 1

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 = iMRT`,

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, `"mol/L"`, molarity, `"M"`.
• `Pi` is the osmotic pressure in `"atm"`, if `R` is the universal gas constant in `"L"cdot"mol/atm"cdot "K"`.
• `T` is temperature in `"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 `Tuarr`, `Piuarr`, etc.