Osmolarity calculator
Osmolarity of a solution (mOsm/L) from mmol/L or g/L, with the van ’t Hoff factor and the comparison with the plasma range.
Result
308.01 mOsm/L
- Calculation
- 9 g/L ÷ 58.44 g/mol = 154 mmol/L
- Calculation
- 154 mmol/L × 2 = 308.01 mOsm/L
Hyperosmolar: above the plasma range, based on the ideal osmolarity.
Calculation, assumptions and sources
OverviewOsmolarity counts dissolved particles: molar concentration times the van ’t Hoff factor.
Osmolarity counts dissolved particles: molar concentration times the van ’t Hoff factor. The concentration is entered in mmol/L, or in g/L with the molar mass, with bench presets. The value is classified against plasma (275 to 295 mOsm/L): hypoosmolar, isoosmolar or hyperosmolar.
Method & formulasosmolarity = C × i
Osmotic pressure depends on the number of particles: one mole of dissolved NaCl releases one mole of Na⁺ and one of Cl⁻, and counts as two.
- Osmolarity
- the concentration of osmotically active particles, in milliosmoles per litre of solution (mOsm/L).
- Van ’t Hoff factor (i)
- the number of particles released per dissolved formula unit: 2 for NaCl, 3 for CaCl₂, 1 for glucose which does not dissociate.
- Osmolarity versus osmolality
- per litre of solution versus per kilogram of water: nearly identical in dilute solution, distinct in plasma where proteins and lipids take up volume.
- Tonicity
- the effective osmotic effect on a cell: only solutes that do not cross the membrane count. Urea has an osmolarity but no tonicity.
Validity domainAn integer i assumes complete dissociation in dilute solution, which strong electrolytes approach; a weak electrolyte (acetic acid) counts between 1 and its maximum.
An integer i assumes complete dissociation in dilute solution, which strong electrolytes approach; a weak electrolyte (acetic acid) counts between 1 and its maximum. At high concentrations the osmotic coefficient drifts from 1 and measurement (freezing-point osmometer) prevails. Urea and ethanol cross membranes and do not count in tonicity.
Physiological salinePhysiological saline is 9 g/L of NaCl, that is 154 mmol/L and, ideally, 308 mOsm/L, above the plasma range.
Physiological saline is 9 g/L of NaCl, that is 154 mmol/L and, ideally, 308 mOsm/L, above the plasma range. In real solution the osmotic coefficient of NaCl is about 0.93 and the effective osmolarity 285 mOsm/L; saline is isotonic.
Reading the resultA hypoosmolar solution swells red blood cells, a hyperosmolar one shrivels them.
A hypoosmolar solution swells red blood cells, a hyperosmolar one shrivels them. For several solutes, osmolarities add up.