# 2: Putting the Os into Osmolarity

We need to make one other change. So far we’ve used the molarity (moles of solute per litre) of solutions, like sugar water, as our way of measuring concentration. In truth, the measure of concentration that we need is not molarity, but osmolarity. Osmolarity is defined by osmoles of solute per litre (kind of like moles, but with an OS on the front). The abbreviation, which we’ll use a lot, is OsM.

So what is an osmole? As you may know, when you dissolve salt in water, it dissociates into Na+ and Cl, in other words, into two pieces. If 1 mole of molecules dissolves into two “pieces” it creates 2 osmoles. If we started with 1 mole of MgCl2, which dissolves into 3 “pieces”, we would get 3 osmoles. Sugar is an organic rather than ionic compound, so it does not dissociate, and 1 mole of sugar dissolves into 1 osmole.  So when you are working out osmolarity the first thing you will need to determine is whether or not the salt in question dissociates into smaller ions.

When we’re talking about diffusion across a membrane (including osmosis), it doesn’t actually matter that salt ions are smaller than sugar molecules, or that their molecular weight is less, or even that the ions are charged while the sugar molecules are electrically neutral. The only thing that actually matters is how many “pieces” the salt dissociates into. For example for a salt “molecule*” that dissociates into two ions, when you dissolve it into water, you will get TWO ions rather than one, and this is what matters for diffusion. Essentially the salt molecule provides twice as many “bits” of dissolved stuff as the sugar molecule, so the osmolarity is also doubled.

### If I dissolve 4 moles of MgCl2 in 2 L of water what will the osmolarity be?

*Yes, I know that salt is an ionic compound and comes in the form of crystalline lattice. However, it is easier to think in terms of a salt “molecule”.