# "Serum osmolarity" vs. "Serum osmolality" - page 2

I'm trying to figure out the difference in definitions between serum osmolarity and serum osmolality. My textbook (Fundamentals of Nursing, 6th edition, by Potter & Perry) states that normal... Read More

1. Quote from ♪♫ in my ♥
Here's how I would look at it:

As stated earlier, a solute is simply one substance that is dissolved in another (the solvent), the result of which is a solution.

The term concentration refers to the relative amounts of the solute and the solvent.

The term molarity refers to the concentration of the solution in terms of the number of moles of solute per liter of solvent.

The term molality refers to the concentration of the solution in terms of the number of moles of solute per liter of solvent.

So, a quick example: Dissolve 30 grams of NaCl in 500 ml of water

The molecular weight of NaCl is 58.4 g/mol. Therefore, 30 grams of NaCl contains (30 g) x (1 mol/58.4 g) = 0.51 mol

The molarity is then (.51 mol) / (0.50 l) = 1.03 mol/l
The molality is then (.51 mol) / (0.50 kg) = 1.03 mol/kg

(Recall that the standard density of water is 1 kg/liter so 500 ml of water has a mass of 500 grams or 0.50 kg)

The terms osmole, osmolarity, and osmolality are related to the above except that you're now looking at the number of particles (typically, ions).

Recall that NaCl is an ionic solid that is completely soluble in water. In other words, each molecule of NaCl that goes into an aqueous solution will dissociate into one Na ion and one Cl ion; each mole of NaCl will dissociate into one mole of Na ions and one mole of Cl ions.

Returning to our example, 30 grams of NaCl = 0.51 moles of NaCl

0.51 moles of NaCl dissociates into 0.51 moles of Na ions and 0.51 moles of Cl ions. In other words, the 0.51 moles of NaCl result in a total of 1.02 moles of ions dissolved in the solution.

Therefore:

osmolarity = (1.02 moles of ions) / (0.5 L) = 2.04 Osm/L
osmolality = (1.02 moles of ions) / (0.5 kg) = 2.04 Osm/kg

Numerically, they are the same but they actually describe different properties of the same solution.

If the solvent were something besides water then the density would be different and the osmolality would be different (but the osmolarity would be the same).

Likewise, if the solvent were water but it was a concentrated solution (lots of stuff dissolved in there), it's density would also be different and the osmolality would be different.

Also recognize that dissolving solutes in a solvent changes the volume a bit so just because you had 500 mL of solvent to start with doesn't mean that you will necessarily have 500 mL of solution. Since the molarity and osmolarity are functions of the solution volume and not the solvent volume, you may get discrepancies between the "arities" and the "alities"

So, there you go. More than you ever really cared to see about some of the fundamentals of solution chemistry.
What's a mole?
2. I too am having that problem of trying to distinguish between the two. I was looking online for an answer and came across this site. The only difference I can distinguish is one is measured against liters, and the other against kilograms. So Perhaps osmolarity is regarded for its volume, and osmolality is for its weight? Hmmm....same thing? I feel really dumb!!
3. Chemistry...yes, one of my favourites.

While i don't specifically remember the difference in definitions of Osmolarity and Osmolality it's helpful to remember that 1 Kg of water equals 1 Litre of water...which explains the OP's similar examples for each definition..
4. Quote from Punch
I too am having that problem of trying to distinguish between the two. I was looking online for an answer and came across this site. The only difference I can distinguish is one is measured against liters, and the other against kilograms. So Perhaps osmolarity is regarded for its volume, and osmolality is for its weight? Hmmm....same thing? I feel really dumb!!
The other distinction, although subtle, is that one involves kg of solvent while the other involves liters of solution. Again, for dilute aqueous solutions it really makes no difference but beyond the subset of biological fluids it can be very important.

As Jag Boy said, 1 L H2O is approximately 1 kg so they're effectively interchangeable.
Last edit by ♪♫ in my ♥ on Mar 28, '08