3: Calculating osmolarity

If you know molarity:

If you need to figure out the osmolarity of a solution and you’re given the molar concentration, its pretty simple, just multiply the molarity by however many “pieces” the solute dissociates into.

If you know the amount of solute:

What if you just know the amount of a solute that was added? For example: what is the approximate osmolarity of the sugar in a can of Red Bull? According to the can which just happens to be sitting next to me, there are 27 g of sugar in 250 mL. The sugar in Red Bull is a mixture of glucose and sucrose (a disaccharide made up of one molecule of glucose and one molecule of fructose bonded together). The molecular weight of sucrose is 342 g. For simplicity, however we will assume here that all the sugar is in the form of glucose. The molecular weight of glucose is 180 g.

First, grams to moles: 27 g x 1 mole/180 g = 0.15 mole

Finally, we know that the sugar molecule remains intact and does not break into smaller bits, so 1 M = 1 OsM.

Now that everything is in the right units, we just need to divide moles by litres to get the osmolarity:

0.15 mole / 0.25 litre = 0.60 OsM.

So the osmolarity of Red Bull is 0.60 OsM, whereas the normal osmolarity of plasma is 0.288 OsM. Think about that next time you have a Red Bull for breakfast!