MOLARITY and OSMOLARITY
The concentration of a solute in a solution is expressed as its “Molarity”. It tells you how much of a solute is
present. The units of molarity are “moles” (mols).
Osmolarity is a concept that allows you to determine if water will move from one side of a membrane to the other
side. That is: will “Osmosis” occur? The units of osmolarity are “osmols” (osm). Water will always move across a
membrane into the solution with the higher osmolarity.
In order to determine a solutions’ osmolarity you will first need to figure out its molarity.
To calculate molarity one needs to understand the following ideas.
1. That 1 Gram-Molecular Weight of a compound is its molecular wt. expressed in grams and that
1 Gm.-Mol. Wt. = 1 mole. i.e., 1 mole of glucose = 180 grams.
2. You will need to know the Mol. Wt. of the solute, the volume of the solution, and how much solute
(in grams) is present in that volume of solution.
3. 1 mole of anything contains 6.02 x 1023 particles of that solute. (Avogadro’s Law)
4. 1 mole of solute per liter of solution = a 1 molar solution = 1M.
5. To calculate the molarity of a solution, use the following equation:
Amount of solute (in grams) per liter
M = ----------------------------------------------------Mol. Wt. of solute
6. To calculate the osmolarity of a solution use the following equation:
OSM = M x the number of particles of dissociation.
For example: NaCl in water dissociates into two particles (Na+ and Cl-) and
MgCl2 in water dissociates into 3 particles (Mg++ and 2 Cl-)
7. Finally, the molarity of our body fluids is about 0.15 M and the osmolarity is about 0.3 Osm
or 300 mOsm.
8. The following are some short-hand notations that are often used.
0.9 g % means: 0.9 grams per 100ml of solution (9 grams per liter)
0.1 g % = 0.1 grams per 100ml (1 gram per liter)
A 5 % glucose solution means that 5 % of the weight of the solution is glucose.
and, 1 liter of water weighs 1000 grams. So, 0.05 x 1000 = 50 grams.
Thus a 5 % solution has 50 grams of solute dissolved in one liter.