Estimate total osmotically active particles from molarity and the number of particles produced by each solute.
Osmolarity = n × COsmolarity estimates the concentration of osmotically active particles in a solution. Multiply molar concentration by the number of particles each molecule produces in solution.
| Symbol | Variable | Description |
|---|---|---|
| n | Dissociation factor | Approximate number of particles produced by one formula unit, such as 2 for NaCl. |
| C | Molar concentration | Solute concentration in mol/L. |
Identify the solute
Decide how many particles the solute contributes. Glucose is often treated as 1, NaCl as 2, and CaCl2 as 3 in simple estimates.
Enter molarity
Use mol/L for the solute concentration. Convert millimolar values to molar before entering them if needed.
Read osmolarity
Use the result as a first-pass estimate for biology, physiology, and cell-culture solution planning.
A 0.154 M NaCl solution is approximately 0.308 Osm/L because sodium chloride contributes about two particles. A 0.300 M glucose solution is approximately 0.300 Osm/L because glucose does not dissociate into ions.
| Solute | Particles (n) | Example result |
|---|---|---|
| NaCl | 2 | 0.154 M → 0.308 Osm/L |
| Glucose or sucrose | 1 | 0.300 M → 0.300 Osm/L |
| CaCl2 | 3 | 0.100 M → 0.300 Osm/L |
| mOsm/L conversion | × 1,000 | 0.308 Osm/L → 308 mOsm/L |
Osmolarity is based on liters of solution, while osmolality is based on kilograms of solvent. Many classroom and quick lab calculations use osmolarity, but clinical and high-precision work often reports osmolality.
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