Question

How is osmolarity related to molarity?

Answer 1

Osmolarity equals molarity times the van't Hoff `i` factor.

Explanation:

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:

`Π = MRT`, where `M` is the molarity and `T` is the Kelvin temperature.

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.

`"NaCl(s)" → "Na"^+("aq") + "Cl"^(-)("aq")`

Thus, 1 mol of `"NaCl"` forms 2 mol of solute particles. It contains 2 Osmol of particles.

The osmotic pressure of a 1 mol/L `"NaCl"` solution will be twice that of a 1 mol/L solution of glucose.

The osmotic pressure formula becomes

`Π = iMRT`, where `i` is the van't Hoff factor.

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 `"NaCl"` has an osmolarity of 2 Osmol/L,

and a solution of 1 mol/L `"CaCl"_2` has an osmolarity of 3 Osmol/L (1 mol `"Ca"^(2+)` and 2 mol `"Cl"^-`.

We can rewrite the formula for osmotic pressure as

`Π = OsM×RT`

If we compare the two formulas, we see that

`OsM = i×M`

That is, the osmolarity is the molarity times the van't Hoff factor.

The video below explains the difference between molarity and osmolarity.