Osmolarity Calculator
Convert molar concentration to osmolarity.
300 mOsm/L — 0.15 M Sodium chloride (NaCl) × i = 2. Real osmolarity is slightly lower (osmotic coefficient < 1).
Osmolarity equals molarity times the number of particles the solute dissociates into, so 0.15 M NaCl → 0.30 osmol/L (300 mOsm/L) because each NaCl splits into 2 ions (Na⁺ and Cl⁻). Glucose stays whole (i = 1), so 0.15 M glucose is only 0.15 osmol/L.
What osmolarity measures
Molarity counts formula units of solute per litre, but osmosis responds to the number of dissolved particles, not molecules. Osmolarity is the concentration of osmotically active particles per litre of solution (osmol/L, usually written as mOsm/L). A solute that stays intact contributes one particle each; a salt that dissociates contributes one particle per ion, so the same molarity can produce very different osmolarities.
i = particles per formula unit (the van’t Hoff factor); osmol/L × 1000 = mOsm/L
Worked example
Physiological saline is 0.15 M NaCl, which dissociates into two ions:
- 1 Find the molar concentration. Saline is about 0.15 mol/L NaCl.
- 2 Count particles per formula unit (i). NaCl → Na⁺ + Cl⁻, so i = 2. Glucose does not dissociate, so i = 1.
- 3 Multiply molarity by i. 0.15 mol/L × 2 = 0.30 osmol/L.
- 4 Convert to mOsm/L if needed. 0.30 osmol/L × 1000 = 300 mOsm/L — close to the osmolarity of blood plasma.
Particles per formula unit (i) for common solutes
Ideal, complete-dissociation values; real osmolarity is a little lower.
| Solute | Dissociation | Particles (i) | 0.1 M → osmol/L |
|---|---|---|---|
| Glucose (C₆H₁₂O₆) | none | 1 | 0.10 |
| NaCl | Na⁺ + Cl⁻ | 2 | 0.20 |
| KCl | K⁺ + Cl⁻ | 2 | 0.20 |
| CaCl₂ | Ca²⁺ + 2 Cl⁻ | 3 | 0.30 |
| MgCl₂ | Mg²⁺ + 2 Cl⁻ | 3 | 0.30 |
| Na₂SO₄ | 2 Na⁺ + SO₄²⁻ | 3 | 0.30 |
Ideal vs real osmolarity, and osmolarity vs osmolality
Ideal vs real. Multiplying by i assumes every formula unit dissociates completely and the ions act independently. In reality ions interact, so the effective particle count is a bit lower. Multiplying i by an osmotic coefficient (φ, slightly below 1) gives the measured value — for example NaCl’s φ is roughly 0.93, so 0.15 M saline measures near 0.28 osmol/L rather than the ideal 0.30.
Osmolarity vs osmolality. Osmolarity is particles per litre of solution (osmol/L); osmolality is particles per kilogram of solvent (osmol/kg). They are nearly equal in dilute aqueous solutions but diverge when the solute is concentrated. Body fluids sit near 300 mOsm/L, which is why 0.30 osmol/L solutions read as roughly isotonic.