How to Calculate the Osmolarity

By Allan Robinson

Osmolarity is the measure of the concentration of a solute in a solution. It is specifically a measure of the number of moles of solute particles in a given volume of solution and is similar to molarity, which measures the number of moles of solute in a given volume of solution. Osmolarity may be calculated from the osmotic coefficient, the number of particles that the solute dissociates into,th and the molarity of the solute.

The Steps

Step 1

Describe the difference between osmolarity and molarity. This distinction is due to the fact that some solutes dissociate when they dissolve, while others don't. For example, table salt (NaCl) dissociates into its component ions (Na+ and Cl-) when it dissolves. On the other hand, glucose does not dissociate into smaller particles when it dissolves.

Step 2

Define the units of osmolarity. Osmolarity is measured in osmoles of solute per liter of solution (osmol/L). An osmole can be informally described as the number of moles of solute components in a solution.

Step 3

Describe the osmotic coefficient. This value is the deviation of a test solution from the ideal solution. The complete calculation of the osmotic coefficient is complex, but it is the degree of dissociation of the solute for simple cases. The osmotic coefficient will therefore have a range of 0 to 1 in these cases, such that the osmotic coefficient will be 1 when the solute is completely dissolved.

Step 4

Calculate the osmolarity from observed values. The osmolarity of a solution may be given as the sum of (yi)(ni)(Ci), where yi is the osmotic coefficient of solute i, n is the number of particles that solute i dissociates into and Ci is the molarity of solute i.

Step 5

Measure osmolarity directly with an osmometer. These devices measure the osmolarity of specific particles, such as those that reduce the vapor pressure of a solution or lower the freezing point of a solution.

About the Author

Allan Robinson has written numerous articles for various health and fitness sites. Robinson also has 15 years of experience as a software engineer and has extensive accreditation in software engineering. He holds a bachelor's degree with majors in biology and mathematics.