How is osmolarity related to molarity?
Osmolarity equals molarity times the van't Hoff
Osmolarity is the number of osmoles of solute per litre of solution .
An osmole is 1 mol of particles that contribute to the osmotic pressure of a solution.
The term comes from the formula for calculating osmotic pressure:
Osmotic pressure is a colligative property. It depends on the number of particles.
If the solute is a nonelectrolyte like glucose, 1 mol of solute forms 1 mol of solute particles.
If the solute is an electrolyte, it will dissociate into ions.
Thus, 1 mol of
The osmotic pressure of a 1 mol/L
The osmotic pressure formula becomes
The van't Hoff factor is the number of moles of particles formed from 1 mol of solute.
Thus, a solution of 1 mol/L
and a solution of 1 mol/L
We can rewrite the formula for osmotic pressure as
If we compare the two formulas, we see that
That is, the osmolarity is the molarity times the van't Hoff factor.
The video below explains the difference between molarity and osmolarity.