Equivalents Calculators

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An equivalent (Eq) is the amount of a substance that reacts with or supplies one mole of H⁺ (in acid-base reactions) or one mole of electrons (in redox reactions), or neutralizes one equivalent of charge (for ions). Milliequivalents (mEq) = mmol × |charge|. In clinical medicine, electrolytes are reported as milliequivalents per liter (mEq/L): plasma sodium 135–145 mEq/L; potassium 3.5–5.0 mEq/L; calcium 4.5–5.5 mEq/L (= 2.25–2.75 mmol/L × 2 charges). Normality (N) = equivalents per liter; used in titrimetry.

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Equivalents and Milliequivalents

For ions: 1 Eq = 1 mol × |charge|. mEq = mmol × |charge|. Na⁺ (charge +1): 140 mEq/L = 140 mmol/L. K⁺ (+1): 4.5 mEq/L = 4.5 mmol/L. Ca²⁺ (+2): 5.0 mEq/L = 2.5 mmol/L. Mg²⁺ (+2): 2.0 mEq/L = 1.0 mmol/L. Cl⁻ (−1): 102 mEq/L = 102 mmol/L.

Normality

Normality (N) = Eq/L. HCl: 1 M = 1 N (donates 1 H⁺/molecule). H₂SO₄: 1 M = 2 N (donates 2 H⁺/molecule). H₃PO₄: 1 M = 3 N. KMnO₄ in acid: 1 M = 5 N (accepts 5 e⁻/molecule).

Clinical Significance

Reporting electrolytes in mEq/L rather than mmol/L is traditional in US clinical medicine (non-US: mmol/L). Advantage: mEq/L reflects charge balance. In normal blood plasma: sum of cation mEq/L ≈ sum of anion mEq/L (electroneutrality). Anion gap = Na⁺ − (Cl⁻ + HCO₃⁻) = 8–12 mEq/L (normal); elevated AG = unmeasured anions → metabolic acidosis diagnosis.

Glossary

Equivalent (Eq)
Amount that reacts with 1 mol H⁺ or transfers 1 mol electrons; for ions: Eq = mol × |charge|; mEq = mmol × |charge|; traditional unit for electrolytes in US clinical medicine.
Milliequivalent (mEq)
mEq = mmol × |charge|; mEq/L used for serum electrolytes; Na⁺ 140 mEq/L = 140 mmol/L; Ca²⁺ 5 mEq/L = 2.5 mmol/L; K⁺ 4.5 mEq/L = 4.5 mmol/L.
Anion Gap
Na⁺ − (Cl⁻ + HCO₃⁻); normal 8–12 mEq/L; elevated > 12 = unmeasured anions → MUDPILES metabolic acidosis; useful in acid-base disorder diagnosis.

Frequently Asked Questions

An equivalent (Eq) is the amount of a substance that: provides or reacts with 1 mole of H⁺ (acid-base); or transfers 1 mole of electrons (redox). For ions: 1 Eq = 1 mol / |charge|. Or: equivalents = moles × |charge|. Examples: Na⁺ (charge +1): 1 mol = 1 Eq. Ca²⁺ (charge +2): 1 mol = 2 Eq. Al³⁺ (charge +3): 1 mol = 3 Eq. SO₄²⁻ (charge −2): 1 mol = 2 Eq. Milliequivalents (mEq) = equivalents × 1,000. Clinically: 140 mEq/L Na⁺ = 140 mmol/L × 1 = 140 mmol/L. 5 mEq/L Ca²⁺ = 5 mmol/L × 2? No: 5 mEq/L = 5/2 = 2.5 mmol/L.

mEq/L = mmol/L × |ionic charge|. mmol/L = mEq/L / |ionic charge|. Examples: Potassium (K⁺, charge = 1): 4.0 mEq/L = 4.0 mmol/L. Calcium (Ca²⁺, charge = 2): 2.5 mmol/L = 2.5 × 2 = 5.0 mEq/L. Magnesium (Mg²⁺, charge = 2): 0.85 mmol/L = 0.85 × 2 = 1.7 mEq/L. Phosphate (HPO₄²⁻, charge = 2): 1.2 mmol/L = 1.2 × 2 = 2.4 mEq/L. For normal plasma: all cation mEq/L must equal all anion mEq/L (electroneutrality principle).

Normality (N) = number of equivalents per liter of solution. It was widely used in analytical chemistry but is now largely replaced by molarity. Relationship: N = M × n (where n = equivalents per formula unit). For H₂SO₄: 1 M → 2 N (donates 2 H⁺). For Na₂CO₃: 1 M → 2 N (accepts 2 H⁺). Still used: titrimetric analysis in some clinical and industrial labs; backcalculation of acid concentration; older reference books and methods. Modern practice: molarity preferred; normality retained for acid-base and redox titrations where it simplifies calculations (N_acid × V_acid = N_base × V_base at equivalence).

In US clinical medicine, electrolytes are traditionally reported in mEq/L. Serum electrolyte reference ranges: Na⁺: 135–145 mEq/L. K⁺: 3.5–5.0 mEq/L. Cl⁻: 98–106 mEq/L. HCO₃⁻: 22–29 mEq/L. Ca²⁺: 4.4–5.4 mEq/L (= 2.2–2.7 mmol/L). Mg²⁺: 1.7–2.5 mEq/L (= 0.85–1.25 mmol/L). Anion gap: AG = Na⁺ − (Cl⁻ + HCO₃⁻); normal 8–12 mEq/L; elevated > 12 mEq/L suggests unmeasured anions — used to diagnose MUDPILES metabolic acidosis (methanol, uremia, DKA, propylene glycol, isoniazid, lactic acidosis, ethylene glycol, salicylates).