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)
Key Notes
- Osmolarity = Σ (concentration × i) in Osm/L: i is the number of particles each formula unit produces when dissolved. Glucose (non-electrolyte): i = 1. NaCl (Na⁺ + Cl⁻): i = 2. CaCl₂ (Ca²⁺ + 2Cl⁻): i = 3. A 1 M NaCl solution has osmolarity ≈ 2 Osm/L.
- Osmolarity vs osmolality: Osmolarity is per liter of solution; osmolality is per kilogram of solvent. For dilute aqueous solutions they are nearly equal, but clinical medicine uses osmolality (measured by freezing point depression) because it is independent of temperature and solute volume.
- Normal blood plasma: 285–295 mOsm/L: The kidneys tightly regulate plasma osmolality. Values above 295 stimulate ADH release (antidiuretic hormone) and thirst. Values below 280 suppress ADH, causing dilute urine. Imbalances cause cells to swell or shrink through osmosis.
- Isotonic, hypertonic, hypotonic: Solutions compared to a reference fluid (usually blood plasma). Isotonic IV fluid (0.9% NaCl ≈ 308 mOsm/L) doesn't cause osmotic shift. Hypertonic draws water out of cells; hypotonic causes cells to swell.
- Applications: Osmolarity is used to select IV fluids in medicine (treating dehydration, hyponatremia, diabetic ketoacidosis), design dialysis solutions, calculate osmotic pressure of food preservation brines, and evaluate kidney concentrating ability in renal function tests.