Direct osmolality is measured by which instrument?

• A. Osmometer
• B. Spectrophotometer
• C. Pycnometer
• D. Refractometer

Answer: (A)

Osmolality measures the concentration of osmotically active particles in a solution, and it is determined directly with an osmometer. Osmometers assess a colligative property of the solution, usually freezing-point depression or vapor-pressure lowering, and convert that measurement into an osmolality value (in mOsm/kg).

Spectrophotometers measure light absorption and are used for concentration based on optical properties, not direct osmolality. Pycnometers determine density by mass and volume, unrelated to osmotic particles. Refractometers assess how light bends to infer concentration, which can estimate solute levels but doesn’t provide a direct osmolality measurement. Thus, the instrument that gives a direct osmolality reading is the osmometer.