Equivalent Weight Calculator

Molar mass is the mass per mole of a substance and is a fixed physical property. Equivalent weight equals molar mass divided by the n-factor and depends on the reaction context. They are equal only when n=1 (e.g., HCl, NaOH, NaCl). For polyprotic acids, polyhydroxyl bases, or multi-electron redox agents, equivalent weight is always smaller than molar mass.

For acids: count the number of H⁺ ions actually donated in the reaction (not necessarily all ionizable protons). For bases: count OH⁻ ions or moles of H⁺ neutralized. For redox: determine the change in oxidation state per formula unit. For salts in precipitation: sum the total positive charge. Always reference the specific balanced chemical equation.

Water (MW = 18.015 g/mol) can act as either an acid (donating H⁺, n=1, EW=18.015) or a base (accepting H⁺, n=1, EW=18.015). In autoionization (2H₂O ⇌ H₃O⁺ + OH⁻), n=1. In rare contexts involving complete decomposition, n=2 (EW=9.008), but this is not a standard acid-base reaction.

Clinical labs report electrolyte concentrations in mEq/L because equivalents represent reactive/physiological capacity. One mEq of Na⁺ (23 mg) and one mEq of Ca²⁺ (20 mg) represent the same electrical charge and similar physiological activity. Normal serum values: Na⁺ 136-145 mEq/L, K⁺ 3.5-5.0 mEq/L, Ca²⁺ 4.5-5.5 mEq/L, Cl⁻ 98-106 mEq/L.

Normality = Molarity × n-factor. For example, 0.1 M H₂SO₄ = 0.2 N (n=2). Conversely, 1 N KMnO₄ in acidic medium = 0.2 M (n=5). This relationship means that at the equivalence point, N₁V₁ = N₂V₂ always holds, regardless of the specific reactants.

In standard acid-base and redox reactions, the n-factor is always a positive integer. However, in some complex reactions or when dealing with average oxidation state changes in mixed-valence compounds, an effective non-integer n-factor can arise. In practice, this is rare and usually indicates the reaction should be broken into simpler steps.

For salts in metathesis (precipitation or complexation) reactions, the n-factor equals the total positive charge (or total negative charge) per formula unit. NaCl: n=1, EW=58.44. CaCl₂: n=2, EW=55.49. AlCl₃: n=3, EW=44.45. Al₂(SO₄)₃: n=6, EW=57.05.

Water hardness is reported in milligrams per liter as CaCO₃ equivalent (EW of CaCO₃ = 100.09/2 = 50.04). Alkalinity is similarly reported. Ion exchange capacity is expressed in equivalents per liter (eq/L). Charge balance calculations use milliequivalents: Σ cation mEq/L should equal Σ anion mEq/L in any water sample.

A milliequivalent (mEq) is one-thousandth of an equivalent. It is calculated as: mEq = mass (mg) / equivalent weight (g/eq), or mEq = mmol × n-factor. In clinical practice, mEq/L is the standard unit for reporting electrolyte concentrations. One mEq of any monovalent ion represents the same electrical charge as one mEq of any other ion.