Osmolality is calculated as Molarity multiplied by what factor?

• A. The number of particles a solute dissociates into
• B. The solvent density
• C. The temperature in Kelvin
• D. The molecular weight of solute

Answer: (A)

Osmolality depends on how many particles are present in solution. To get from molarity to osmolality, you multiply by the factor that describes how many particles the solute produces when it dissolves. This factor is the van’t Hoff factor, the number of particles into which each solute molecule dissociates.

If a solute dissociates into more particles, the osmolality increases accordingly. For example, a non-electrolyte like glucose doesn’t dissociate, so it contributes one particle per mole (≈1 Osm/L per mole). A salt such as NaCl dissociates into two particles (Na+ and Cl−), contributing roughly twice as many osmol units per mole (≈2 Osm/L per mole) when fully dissociated. Incomplete dissociation would yield a smaller factor.

The other options don’t determine osmolality: solvent density and molecular weight aren’t used in the basic calculation, and temperature isn’t part of the standard multiplier, though extreme conditions can affect dissociation somewhat.