Osmolarity
Solute particle concentration in solution.
Formula
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Example
NaCl: n=2
How it works
Osmolarity tells you how many dissolved particles are present in a liter of solution, which matters because particles drive water movement across membranes. The formula works by multiplying the molar concentration of a solute by the number of particles each formula unit produces in solution; for example, one molecule that stays intact counts as 1 particle, while a dissociating salt may count as 2 or more. This makes osmolarity a better predictor of osmotic effect than molarity when solutes split apart in water.
When to use it
Use this when comparing the tonicity or water-pulling effect of biological fluids, IV solutions, or lab buffers. It is especially useful in physiology, chemistry, and medicine whenever a solute may dissociate into multiple ions.
Worked example, step by step
A 0.20 M solution of glucose is prepared. Glucose does not dissociate, so n = 1. Apply the formula: Osm = n × C Osm = 1 × 0.20 mol/L Osm = 0.20 Osm/L So the solution has an osmolarity of 0.20 Osm/L, or 200 mOsm/L.
Common mistakes to avoid
- using molarity directly without accounting for dissociation
- counting the wrong number of particles for an ionic compound
- forgetting that osmolarity is based on particles in solution, not just formula units
Did You Know?
Osmolarity governs how water flows across cell membranes, which is why IV fluids must be carefully balanced.
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