osmolality ...or...osmolarity!!

This probably wont interest many of us.But it is interesting to know why ,how and when we use these terms!

Before going to osmolality or osmolarity, lets get the definition of molality & molarity.

High school physics - Mole is a SI unit for amount of substance i.e quantity. The no. of atoms in one mole is Avagadro's consatant and we dont need that here!!

MOLALITY - no. of moles per kg of a solution (mol /kg)

MOLARITY - no. of moles per litre of a solution (mol/L)

So if we apply this ..... Osmolality is no. of osmoles /kg, and Osmolarity is no. of osmoles /L .

One more word we need to remember- DENSITY which is mass / volume.

Lets quickly discuss abt water which has a density of 1. To make it simpler....1L of water will be equal to 1kg of water! So if you apply the definition osmolarity & osmolality to water, then they both would be the same.

Since blood has a density close to water i.e 1 (we know that because the specific gravity of blood is 1.03 - 1.05), now we can say that its osmolality is equal to its osmolarity, and sooooooo they can both be used interchangeably.

Osmolarity & Osmolality can be used interchangeably in case of blood/plasma, only beacuse its density is nearly equal to 1. For fluids whose density is not near 1, then you cannot use these terms to mean the same thing, as osmolality & osmolarity will differ.

Is it still not clear ? !