The formula
Osmolarity counts dissolved particles, not molecules. For each solute, multiply its molar concentration by the number of particles it splits into, i, and add the results:
osmolarity (Osm/L) = Σ (molarity × i)
| Solute | Particles (i) |
|---|---|
| Glucose, sucrose, urea, glycerol | 1 |
| NaCl, KCl, NaHCO3 | 2 |
| CaCl2, MgCl2, Na2SO4 | 3 |
Results are usually given in milliosmoles per liter, mOsm/L.
Example: 0.9% saline
- 0.9% w/v is 9 g per liter.
- 9 ÷ 58.44 g/mol = 0.154 mol/L, or 154 mM.
- NaCl splits into 2 ions: 154 × 2 = 308 mOsm/L.
Example: 5% dextrose
50 g/L ÷ 180.16 g/mol = 0.278 mol/L. Glucose does not dissociate, so the osmolarity is 278 mOsm/L.
Example: plasma
Clinically, serum osmolality is estimated from sodium, glucose and urea:
2 × Na (mmol/L) + glucose (mg/dL) ÷ 18 + BUN (mg/dL) ÷ 2.8
With Na 140, glucose 90 mg/dL and BUN 14 mg/dL: 280 + 5 + 5 = 290 mOsm/kg. In mmol/L units, use 2 × Na + glucose + urea. The normal range is about 275–295. A measured value well above the estimate, an osmolal gap over about 10, points to an unmeasured solute such as ethanol, methanol or ethylene glycol.
Why measured values are lower
Ions in solution do not act fully independently, so 0.9% saline behaves like about 286 rather than 308 mOsm. The ratio is the osmotic coefficient, about 0.93 for NaCl at this strength. The formula above gives the ideal value; an osmometer gives the real one.
Osmolarity and osmolality
Osmolarity is per liter of solution and osmolality per kilogram of water. In dilute solutions such as plasma they are nearly the same number. Osmometers measure osmolality, which does not change with temperature.
Tonicity
A solution's osmolarity says how many particles it holds; its tonicity says what it does to cells. Solutes that cross the membrane, such as urea, count toward osmolarity but not tonicity. 5% dextrose is iso-osmotic when infused, but becomes hypotonic once cells take up the glucose.
Calculators
The Osmolarity Calculator adds up any mix of solutes. Turn grams per liter into molarity first with the Molarity Calculator, or a percentage with the Percent Solution Calculator.