Chemistry · Concentration
Osmolarity calculator.
Ideal osmolarity from concentration and particles per formula unit, with the osmotic pressure at your temperature. Education and lab-prep only.
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Osmolarity formula
Osmolarity is the number of osmoles of solute particles per litre of solution: osmolarity = sum of (n x C), where C is the molar concentration of each solute and n is the number of particles each formula unit gives (NaCl 2, CaCl2 3, glucose 1). The full expression multiplies each term by an osmotic coefficient phi that describes non-ideal behaviour; this tool uses phi = 1, the ideal case. Osmolality is the same idea per kilogram of solvent.
The van't Hoff equation for a dilute ideal solution gives the osmotic pressure Pi = osmolarity x R x T, with R = 8.314 462 618 J/(mol K) and the osmolarity in mol/m3.
Worked example. 0.15 mol/L NaCl: 0.15 x 2 = 0.300 Osm/L, or 300 mOsm/L ideal. At 25 C (298.15 K): 300 mol/m3 x 8.314 x 298.15 = 743.7 kPa, about 7.34 atm. A measured osmolality will differ because of the osmotic coefficient.
This is a calculation aid for chemistry teaching and bench solution prep. It is not for preparing intravenous, injectable or other clinical fluids, and it is not medical advice.
Frequently asked questions
How do I calculate osmolarity?
Multiply each solute concentration (mol/L) by the particles it gives per formula unit, then add them up. 0.15 M NaCl is 0.15 x 2 = 0.30 Osm/L.
What is the difference between osmolarity and osmolality?
Osmolarity is osmoles per litre of solution. Osmolality is osmoles per kilogram of solvent.
Why does my measured value differ from the calculated one?
The calculation assumes an osmotic coefficient of 1 (ideal). Real salts interact, so the measured value is usually somewhat different.
Can I use this to prepare IV or clinical fluids?
No. It is an education and bench-prep calculator only. Clinical solutions must be made and checked under qualified professional procedures.
Sources
Checked
- Wikipedia: Osmotic concentration (osmolarity = sum of phi x n x C)
- Wikipedia: Osmotic pressure (van 't Hoff equation, valid for dilute ideal solutions)
- NIST CODATA: molar gas constant R
- NIST SP 811, Appendix B.8 (1 atm = 101 325 Pa)
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Lab safety and scope
This is arithmetic, not professional advice. Check every result against your own procedure, the safety data sheet (SDS) for each chemical, and your lab supervisor or instructor before you prepare, mix or dispose of anything. Verify values on the reagent bottle, since purity and hydration state change the real molar mass.