0.308
Osm/L
308
mOsm/L
0.308
Osm/L
308
mOsm/L
The Osmolarity Calculator determines the osmolar concentration of a solution based on the molarity, the van't Hoff factor (number of particles produced upon dissociation), and the number of solute types. Osmolarity is critical in physiology and medicine for understanding water movement across cell membranes and for preparing intravenous solutions.
Enter the molarity, the van't Hoff factor (i = 1 for non-electrolytes like glucose, i = 2 for NaCl, i = 3 for CaCl2), and the number of particle types to obtain the osmolarity in both Osm/L and mOsm/L. Normal human blood osmolarity is approximately 275-295 mOsm/L.
Osmolarity is calculated as:
Osmolarity = M x i x n
Where M is the molarity, i is the van't Hoff factor (the number of particles each formula unit produces in solution), and n is the number of different solute types (usually 1 for a single solute).
For NaCl (i = 2, since it dissociates into Na⁺ and Cl⁻), a 0.15 M solution has osmolarity = 0.15 x 2 x 1 = 0.30 Osm/L = 300 mOsm/L, which is approximately isotonic with blood.
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Normal saline (0.154 M NaCl) has osmolarity of approximately 308 mOsm/L, which is close to the physiological range of 275-295 mOsm/L.
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5% dextrose (glucose, i = 1 as a non-electrolyte) has osmolarity of 278 mOsm/L, approximately isotonic with blood.
The van't Hoff factor (i) represents the number of particles a solute produces when dissolved. Non-electrolytes like glucose have i = 1 (they do not dissociate). Strong electrolytes like NaCl have i = 2 (Na⁺ + Cl⁻), CaCl2 has i = 3 (Ca²⁺ + 2 Cl⁻), and Na2SO4 has i = 3 (2 Na⁺ + SO4²⁻). In practice, i may be slightly less than the theoretical value due to ion pairing.
Osmolarity is measured per liter of solution (Osm/L), while osmolality is measured per kilogram of solvent (Osm/kg). For dilute aqueous solutions like body fluids, the two values are nearly identical. Osmolality is technically more precise for clinical use because it does not depend on temperature or volume, but osmolarity is more commonly used in calculations.
Osmolarity determines the direction of water movement across cell membranes by osmosis. Isotonic solutions (approximately 290 mOsm/L) do not cause cells to swell or shrink. Hypotonic solutions cause cells to swell, and hypertonic solutions cause cells to shrink. This is critical for preparing IV fluids, managing electrolyte disorders, and understanding kidney function.
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