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Osmolarity Calculator

Calculate the osmolarity of a solution in mOsm/L from each solute's concentration, number of particles and osmotic coefficient, with the osmotic pressure at your temperature.

Formula

osmolarity=∑ici ni ϕi\text{osmolarity} = \sum_i c_i\, n_i\, \phi_i
Π=osmolarity×R T\Pi = \text{osmolarity} \times R\,T
particles\text{particles}
ions or molecules formed per formula unit: 2 for NaCl, 3 for CaCl₂, 1 for glucose
ϕ\phi
how far real behaviour departs from ideal; 1 is ideal

How it works

Osmolarity counts the number of dissolved particles per litre, because osmotic effects depend on the number of particles and not on their kind. A mole of sodium chloride gives two osmoles, and a mole of glucose one. Adding up the contributions of every solute gives the osmolarity. For a solute that does not behave ideally, the osmotic coefficient scales its contribution.

The osmotic pressure of the solution follows from the van 't Hoff relationship. Osmolarity is per litre of solution, while an osmometer reports osmolality, per kilogram of solvent; the two are close for dilute solutions. Osmolarity is also not tonicity: a solute that freely crosses the cell membrane adds to the osmolarity but does not draw water out of cells.

Worked example

154 mM NaCl (physiological saline) with 5 mM glucose, at 37 °C.

  1. NaCl: 154 mM × 2 × 1 = 308 mOsm/L.
  2. Glucose: 5 mM × 1 × 1 = 5 mOsm/L.
  3. π = 0.313 mol/L × 8.314 J/(mol·K) × 310.15 K = 807 kPa.

313 mOsm/L, an osmotic pressure of 807 kPa (7.97 atm). The real osmolality of 0.154 M NaCl is nearer 286 mOsm/kg, because the osmotic coefficient is about 0.93.

These are the values the calculator opens with, so you can check its output against this example.

Assumptions

  • Complete dissociation of the electrolytes.
  • The osmotic coefficients you enter; with 1, the ideal value.
  • Osmolarity, not osmolality and not tonicity.

Common mistakes

  • Forgetting that a salt counts as more than one particle.
  • Comparing an ideal calculation with a measured osmolality without allowing for the osmotic coefficient.
  • Calling a solution isotonic because its osmolarity matches cells, when it contains a permeant solute.

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Related equipment

Service documentation, failure modes and parts for the instruments this calculation is used with.