Osmolarity = Σ (Molarity × n particles)
Osmolarity measures the total concentration of dissolved particles in a solution (mOsm/L). Plasma osmolarity ≈ 280-310 mOsm/L. Calculated as: Osm = Σ(Molarity × dissociation factor). NaCl dissociates into 2 particles (Na⁺ + Cl⁻), so 0.9% NaCl (0.154 M) gives 308 mOsm/L — isotonic with blood. Hypotonic solutions (<250) cause cells to swell; hypertonic (>310) cause shrinkage. Common IV fluids: Normal Saline 308 mOsm/L (isotonic), D5W 252 (slightly hypotonic once glucose metabolized), Lactated Ringer's 273, 3% NaCl 1026 (hypertonic). Osmolarity is critical in IV fluid selection, TPN formulation, and understanding kidney function.