4: Osmolarity adds up

It can take many solutes to make osmolarity

On the last screen, we calculated the osmolarity of the sugar in Red Bull to be 0.60 OsM. That is not, however, the same thing as the osmolarity of Red Bull. The reason is that sugar is not the only thing dissolved in that can. For example, there are also 35mg of salt. To calculate the total osmolarity, we also need to take the salt into account (as well as a bunch of other minor players like color and flavor molecules, but we’ll stick with the salt for now).

In order to figure out how many moles of salt we have, we need to know the molecular weight. One mole of sodium weighs 23 g, and one mole of chlorine weighs 35.4 g. When the sodium and chlorine are combined in a single molecule, one mole of salt weighs 23 + 35.4 = 58.4 g.

So, converting from mg to moles: 35.4 mg x 1 g / 1000 mg x 1 mole / 58.4 g = 0.00060 mole

And we know that salt dissociates into one sodium ion and one chlorine ion (Na⁺ and Cl^–), making TWO moles of ions. Or,

            0.00060 undissolved mole –> 0.0012 mole of dissolved ions

And, remembering that there are 0.25L the osmolarity associated with the salt is:

            0.0012 osmole / 0.25 litre = 0.0048 OsM

To get the total osmolarity of the Red Bull, we add up the two osmolarities associated with each ingredient:

            total osmolarity = 0.60 OsM + 0.0048 OsM = 0.6048 OsM.

Obviously we could keep going down the ingredient list, but probably you’ve got the idea by now.