What does osmolarity measure?

Osmolarity specifically measures the concentration of solute particles in a solution. It reflects the number of osmotic particles—ions, molecules, or particles that result from the dissociation of solutes—per liter of solvent. Higher osmolarity indicates that there are more solute particles in a given volume of solvent, which is critical for understanding how substances interact within biological systems, especially in relation to fluid balance and cellular function.

This concept is particularly important in the context of bodily fluids, as osmolarity helps to regulate water movement across cell membranes via osmosis. When cells are in solutions with varying osmolarities—such as isotonic, hypertonic, or hypotonic solutions—understanding osmolarity can help predict how water will flow in or out of the cells, affecting hydration and metabolic processes.

In contrast, the other options focus on different characteristics of solutions. The weight of solutes does not account for how those solutes might dissociate into different particles in solution, and thus wouldn’t accurately represent osmolarity. Temperature and volume of the solvent are also unrelated to the specific measurement of particle concentration that defines osmolarity.