Osmolarity and osmolality are units of solute concentration that are often used in reference to biochemistry and body fluids. While any polar solvent could be used, these units are used almost exclusively for aqueous (water) solutions. Learn what osmolarity and osmolality are and how to express them.

### Osmoles

Both osmolarity and osmolality are defined in terms of osmoles. An osmole is a unit of measurement that describes the number of moles of a compound that contribute to the osmotic pressure of a chemical solution.

The osmole is related to osmosis and is used in reference to a solution where osmotic pressure is important, such as blood and urine.

### Osmolarity

Osmolarity is defined as the number of osmoles of solute per liter (L) of a solution. It is expressed in terms of osmol/L or Osm/L. Osmolarity depends on the number of particles in a chemical solution, but not on the identity of those molecules or ions.

### Sample Osmolarity Calculations

A 1 mol/L NaCl solution has an osmolarity of 2 osmol/L. A mole of NaCl dissociates fully in water to yield two moles of particles: Na^{+} ions and Cl^{-} ions. Each mole of NaCl becomes two osmoles in solution.

A 1 M solution of sodium sulfate, Na_{2}SO_{4}, dissociates into 2 sodium ions and 1 sulfate anion, so each mole of sodium sulfate becomes 3 osmoles in solution (3 Osm).

To find the osmolarity of a 0.3% NaCl solution, you first calculate the molarity of the salt solution and then convert the molarity to osmolarity.

Convert percent to molarity:

0.03 % = 3 grams / 100 ml = 3 grams / 0.1 L = 30 g/L

molarity NaCl = moles / liter = (30 g/L) x (1 mol / molecular weight of NaCl)

Look up the atomic weights of Na and Cl on the periodic table and add the together to get the molecular weight. Na is 22.99 g and Cl is 35.45 g, so the molecular weight of NaCl is 22.99 + 35.45, which is 58.44 grams per mole. Plugging this in:

molarity of the 3% salt solution = (30 g/L) / (58.44 g/mol)

molarity = 0.51 M

You know there are 2 osmoles of NaCl per mole, so:

osmolarity of 3% NaCl = molarity x 2

osmolarity = 0.51 x 2

osmolarity = 1.03 Osm

### Osmolality

Osmolality is defined as the number of osmoles of solute per kilogram of solvent. It is expressed in terms of osmol/kg or Osm/kg.

When the solvent is water, osmolarity and osmolality may be nearly the same under ordinary conditions, since the approximate density of water is 1 g/ml or 1 kg/L. The value changes as the temperature changes (e.g., the density of water at 100 C is 0.9974 kg/L).

### When to Use Osmolarity vs Osmolality

Osmolality is convenient to use because the amount of solvent remains constant, regardless of changes in temperature and pressure.

While osmolarity is easy to calculate, it's less difficult to determine because the volume of a solution changes according to temperature and pressure. Osmolarity is most commonly used when all measurements are made at a constant temperature and pressure.

Note a 1 molar (M) solution will usually have a higher concentration of solute than a 1 molal solution because solute accounts for some of the space in the solution volume.