MathBench > Cellular Processes


It can take many solutes to make osmolarity

On the last screen, we calculated the osmolarity of a classic coke to be 0.49 OsM. That is not, however, exactly right. 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 11g, and one mole of chlorine weighs 17g. When the sodium and chlorine are combined in a single molecule, one mole of the stuff weighs 11+17 = 28g.

So, converting from mg to moles: 35 mg * 1 g / 1000 mg * 1 mole / 28g = 0.00125 moles

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

0.00125 undissolved moles --> 0.00250 moles of dissolved ions

And, remembering that the 12 ounces of coke was the same as 0.42 liters, the osmolarity associated with the salt is:

0.0025 osmoles / 0.42 liters = 0.006 OsM

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

total osmolarity = 0.49 OsM + 0.006 OsM = 0.496 OsM.

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

What is the osmolarity of seawater given the following:

Solute g / L molecular weight
Cl- 19 35
Na+ 10.5 23
Mg 1.3 24
S 0.8 32

(To make this problem interactive, turn on javascript!)

I think I have the answer: 19 * 1/35 + 10.5 * 1/23 + 1.3 *
1/24 + .8 * 1/32
= 0.54 + 0.46 + 0.054 + 0.025
= 1.08 OsM = 1080 mOsM