1 mole NaCl equals how many osmoles when dissolved in water?

• A. 1
• B. 2
• C. 3
• D. 0.5

Answer: (B)

Osmolarity counts the total number of osmotically active particles in solution. Sodium chloride dissociates into two ions, Na+ and Cl−, so each mole of NaCl yields two particles that contribute to the osmotic effect. Under ideal conditions, the van't Hoff factor for NaCl is about 2, meaning 1 mole of NaCl corresponds to 2 osmoles. In dilute solutions this approximation holds well, since there is near-complete dissociation.