How can I calculate osmolarity of blood?
You multiply the molarity of each solute by the number of osmoles that it produces.
Then you add them together.
An osmole (Osm) 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: one mole of Na⁺ and one mole of Cl⁻.
A solution of 1 mol/L NaCl has an osmolarity of 2 Osm/L.
A solution of 1 mol/L CaCl₂ has an osmolarity of 3 Osm/L (1 mol Ca²⁺ and 2 mol Cl⁻).
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.
[Na⁺] = 0.140 mol/L. But, each Na⁺ ion pairs with a negative ion X⁻ such as Cl⁻ to give 2 Osm of particles.
∴ NaX osmolarity =
Glucose osmolarity =
BUN osmolarity =
∴ Blood osmolarity = (0.280 + 0.008 32 + 0.0071) Osm/L = 0.295 Osm/L = 295 mOsm/L