Osmolarity & Tonicity
Computes osmolarity (mOsm/L) and tonicity for IV solutions and cell-culture media, accounts for the dissociation factor (i). For dilute aqueous solutions osmolarity ≈ osmolality.
How to use this tool
Work out the total osmotic strength of a solution from the solutes in it, and whether it lands close to the tonicity of blood. List each solute with its concentration; salts that split into ions count for more.
What to enter
- Solute: type or pick a name; choosing a preset (NaCl, CaCl₂, glucose…) auto-loads its i factor.
- Conc.: concentration of that solute in millimolar (mM).
- i factor: particles released per formula unit (van 't Hoff): NaCl = 2, CaCl₂ = 3, glucose = 1.
- + Add solute: stack as many components as your recipe contains.
Reading the result
The headline is total osmolarity in mOsm/L (the sum of i × concentration across all solutes), with a tonicity verdict against plasma (~290 mOsm) and a per-solute breakdown. Hypotonic / isotonic / hypertonic tells you how cells would respond.
Worked example
154 mM NaCl with i = 2 gives 308 mOsm/L, which reads as ≈ isotonic, exactly the 0.9% saline used for IV drips.
Total Osmolarity
Total osmolarity sums i × concentration over every solute, so dissociating salts (NaCl i=2, CaCl₂ i=3) count more than sugars (i=1). Tonicity compares against plasma (~290 mOsm): hypertonic solutions draw water out of cells, hypotonic ones make them swell.
Methodology
Each dissolved solute contributes i · C osmoles, where i is the van 't Hoff factor (number of particles per formula unit) and C is its molar concentration. Total osmolarity is the sum across solutes, reported in mOsm/L. For dilute aqueous solutions osmolarity (per litre) and osmolality (per kg water) are nearly equal; the tool labels the result osmolarity since it works from volume.
Reference ranges
- Normal human serum: ≈ 275–295 mOsm/kg.
- Isotonic saline (0.9% NaCl): ≈ 308 mOsm/L. D5W: ≈ 252 mOsm/L.