Milliequivalent (mEq), Millimole & Milliosmole Practice
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These three units describe amounts of electrolytes. Millimoles (mmol) measure the number of molecules, milliequivalents (mEq) account for ionic charge (valence), and milliosmoles (mOsm) count the total dissociated particles that determine osmotic pressure.
Formulasmmol = mg ÷ molecular weight • mEq = mmol × valencemOsm/L = (grams per liter ÷ MW) × number of particles × 1,000
Worked example
How many mEq are in 1 g of KCl (MW 74.5, valence 1)? mmol = 1,000 ÷ 74.5 = 13.4; mEq = 13.4 × 1 = 13.4 mEq. Osmolarity of 0.9% NaCl = 9 g/L ÷ 58.5 × 2 × 1,000 = 308 mOsm/L.
💡 Valence trap: monovalent ions (Na⁺, K⁺) → mEq = mmol; divalent ions (Ca²⁺, Mg²⁺) → 2 mEq per mmol.