Osmolarity Calculator
Calculate the osmolarity of a solution using molarity and the van't Hoff factor, or combine multiple solutes to estimate total solution osmolarity.
Calculate Solution Osmolarity
Choose a single-solute calculation or add multiple solutes.
Osmolarity (Osm/L) = M × i
Total Osmolarity = Σ(M × i)
What Is Osmolarity?
Osmolarity describes the concentration of osmotically active particles in a solution. It is commonly expressed as osmoles per liter of solution (Osm/L) or milliosmoles per liter (mOsm/L).
Unlike molarity, which counts the amount of dissolved substance, osmolarity accounts for the number of dissolved particles represented by the van't Hoff factor.
Osmolarity Formula
For a simple solution, estimated osmolarity can be calculated by multiplying the solute molarity by its van't Hoff factor.
How to Use the Osmolarity Calculator
Enter Molarity
Enter the concentration of the solute in mol/L or mmol/L.
Enter the Factor
Add the appropriate van't Hoff factor for the solute under the conditions being modeled.
Calculate
The calculator returns the estimated osmolarity in Osm/L and mOsm/L.
Osmolarity Calculation Example
Suppose a hypothetical solution contains a solute at 0.10 mol/L and the chosen van't Hoff factor is 2. Multiply the molarity by the factor.
Molarity vs Osmolarity
| Property | Molarity | Osmolarity |
|---|---|---|
| What it represents | Amount of solute | Osmotically active particle concentration |
| Common unit | mol/L | Osm/L |
| Particle factor | Not directly included | Included through i |
| Basic relationship | M | M × i |
What Is the van't Hoff Factor?
The van't Hoff factor, represented by i, describes the effective number of dissolved particles associated with each formula unit of a solute.
In an idealized calculation, a non-dissociating solute can use a factor of 1. A solute modeled as producing two particles per formula unit can use a factor of 2. Real solutions may behave non-ideally, so experimental effective values can differ from simple theoretical counts.
i = 1
Represents an idealized solute treated as remaining one dissolved particle per formula unit.
i = 2
Represents an idealized case where each formula unit is treated as producing two dissolved particles.
Real Solutions
Interactions between dissolved species can cause actual osmotic behavior to differ from an ideal integer particle count.
How to Calculate Osmolarity with Multiple Solutes
For an idealized mixture, calculate the osmolarity contribution of each solute separately and add the contributions together.