Final answer:
Tonicity describes how a solution can change the volume of a cell and often correlates with osmolarity. Osmolality measures the ratio of solutes to solvent in a solution and reflects a person's hydration status.
Step-by-step explanation:
Tonicity describes how an extracellular solution can change the volume of a cell by affecting osmosis. A solution's tonicity often directly correlates with the osmolarity of the solution. Osmolarity describes the total solute concentration of the solution. A solution with low osmolarity has a greater number of water molecules relative to the number of solute particles; a solution with high osmolarity has fewer water molecules with respect to solute particles.
Osmolality is the ratio of solutes in a solution to a volume of solvent in a solution. Plasma osmolality is the ratio of solutes to water in blood plasma. A person's plasma osmolality value reflects his or her state of hydration.