Answer
Osmolarity is the measure of a solute concentration. Tonicity is the ability of a solution to affect the fluid volume and pressure of the cell. The key difference between the two is that osmolarity refers of one solution, while tonicity describe the relationship between two solutions separated by a membrane.
Work Step by Step
Osmotic pressure is the pressure created by water moving across a membrane due to osmosis. Osmolarity is the measure of the osmotic pressure of a given solution expressed in osmol/liter. Osmolarity is used more often because it is easier to calculate the volume of a cell that the weight of water present in that cell. Tonicity is the measure of the osmotic pressure of two solutions separated by a permeable membrane expressed in mol/liter. There are three types of solutions:hypotonic solutions, hypertonic solutions, and isotonic solutions. Another major difference between the two terms is that osmolarity measures the solutes that both penetrate and don't penetrate the membrane, while tonicity measures only the solutes that don't penetrate because of the selective permeability of the membrane.