An osmole is 1 mol of particles that contribute to the osmotic pressure of a solution.
For example, NaCl dissociates completely in water to form Na⁺ ions and Cl⁻ ions.
Thus, each mole of NaCl becomes two osmoles in solution: 1 mol of Na⁺ and 1 mol of Cl⁻.
A solution of 1 mol/L NaCl has an osmolarity of 2 Osmol/L.
A solution of 1 mol/L CaCl2 has an osmolarity of 3 Osmol/L
(1 mol Ca2+ and 2 mol Cl-).
EXAMPLE
Calculate the osmolarity of blood.
The concentrations of solutes are: [Na+] = 0.140 mol/L; [glucose] = 180 mg/100 mL;
[BUN] (blood urea nitrogen) = 20 mg/100 mL.
Solution
[Na+] = 0.140 mol/L.
But, each Na+ ion pairs with a negative ion X- such as Cl- to give 2 Osmol of particles.
∴ NaX osmolarity=0.140mol1lL×2lOsmol1mol=0.280 Osmol/L
Glucose osmolarity=0.150g100mL×1000mL1lL×1mol180.2g×1lOsmol1mol=0.008 32 Osmol/L
BUN osmolarity=0.020g100mL×1000mL1lL×1mol28.01g×1lOsmol1mol=0.0071 mol/L
∴ Blood osmolarity=(0.280 + 0.008 32 + 0.0071) Osmol/L
=0.295 Osmol/L=295 mOsmol/L
Here's a video on calculating osmolarity.
