Osmolarity Formula
Osmolarity measures total solute particle concentration in a solution.
Calculated as n × M, where n is dissociation factor and M is molarity.
The Formula
Osmolarity measures the total concentration of all solute particles in a solution. It accounts for the fact that some solutes dissociate into multiple particles when dissolved.
For example, NaCl splits into Na⁺ and Cl⁻ in water, producing 2 particles per formula unit. Glucose does not dissociate, so it produces only 1 particle per molecule.
Variables
| Symbol | Meaning |
|---|---|
| Osmolarity | Total solute particle concentration (in osmol/L or mOsm/L) |
| n | Number of particles each solute dissociates into (dimensionless) |
| C | Molar concentration of each solute (in mol/L) |
| Σ | Sum across all solutes in the solution |
Common Dissociation Factors
| Solute | Dissociation | n |
|---|---|---|
| Glucose (C₆H₁₂O₆) | Does not dissociate | 1 |
| NaCl | Na⁺ + Cl⁻ | 2 |
| CaCl₂ | Ca²⁺ + 2Cl⁻ | 3 |
| Na₂SO₄ | 2Na⁺ + SO₄²⁻ | 3 |
| Urea | Does not dissociate | 1 |
Example 1
Calculate the osmolarity of 0.9% saline (0.154 M NaCl).
NaCl dissociates into 2 particles: Na⁺ and Cl⁻
Osmolarity = n × C = 2 × 0.154
Osmolarity = 0.308 Osm/L = 308 mOsm/L (approximately isotonic with blood plasma)
Example 2
A solution contains 0.1 M glucose and 0.05 M CaCl₂. What is the total osmolarity?
Glucose: n = 1, contribution = 1 × 0.1 = 0.1 Osm/L
CaCl₂: n = 3, contribution = 3 × 0.05 = 0.15 Osm/L
Total osmolarity = 0.1 + 0.15
Total osmolarity = 0.25 Osm/L = 250 mOsm/L
When to Use It
Osmolarity is critical in medical and biological contexts.
- Determining if an IV solution is isotonic, hypertonic, or hypotonic
- Calculating osmotic pressure across cell membranes
- Preparing laboratory solutions for cell culture
- Clinical assessment of blood and urine osmolarity
- Understanding fluid balance in the human body (normal plasma osmolarity is 275–295 mOsm/L)