Acids, Bases & Salts — 60 Terms Defined

Diprotic Acid Acids, Bases & Salts mol/L An acid that can donate two protons per molecule. H₂A ⇌ 2H⁺ + A²⁻

Double Salt Acids, Bases & Salts Dimensionless A salt containing two different cations or anions that dissociates completely into all its constituent ions in solution. e.g. K₂SO₄·Al₂(SO₄)₃·24H₂O

End Point Acids, Bases & Salts mL The point in a titration at which the indicator changes colour, signalling that the reaction is judged complete.

Equivalence Point Acids, Bases & Salts mol The point in a titration at which exactly stoichiometric amounts of the two reactants have been combined. n_acid × basicity = n_base × acidity

Henderson–Hasselbalch Equation Acids, Bases & Salts Dimensionless The equation giving the pH of a buffer from the pKa of its acid and the ratio of conjugate base to acid. pH = pKa + log([A⁻]/[HA])

Hydrolysis Constant Acids, Bases & Salts mol/L The equilibrium constant for the hydrolysis of a salt ion in water. Kh = Kw / Ka (anion)

Indicator Range Acids, Bases & Salts Dimensionless The span of pH over which an indicator's colour visibly changes, roughly one unit either side of its pKa. pH = pK_In ± 1

Ionic Product of Water Acids, Bases & Salts mol²/L² The equilibrium constant for the self-ionisation of water, equal to the product of the hydrogen and hydroxide ion concentrations. Kw = [H⁺][OH⁻] = 1.0 × 10⁻¹⁴

Levelling Effect Acids, Bases & Salts Dimensionless The tendency of a solvent to reduce all acids or bases stronger than its own ionised forms to the same effective strength.

Lewis Theory Acids, Bases & Salts Dimensionless The theory defining an acid as an electron pair acceptor and a base as an electron pair donor. A + :B → A←B

Methyl Orange Acids, Bases & Salts Dimensionless An acid–base indicator that is red in acid and yellow in alkali, changing between pH 3.1 and 4.4. C₁₄H₁₄N₃NaO₃S

Monoprotic Acid Acids, Bases & Salts mol/L An acid that can donate only one proton per molecule. HA ⇌ H⁺ + A⁻

Neutral Salt Acids, Bases & Salts Dimensionless A salt whose aqueous solution has a pH of 7 because neither of its ions hydrolyses appreciably.

Neutral Solution Acids, Bases & Salts mol/L A solution in which the concentrations of hydrogen and hydroxide ions are exactly equal. [H⁺] = [OH⁻] = √Kw

Ostwald's Dilution Law Acids, Bases & Salts mol/L The relation showing how the degree of ionisation of a weak electrolyte increases on dilution. Ka = cα² / (1 − α)

Oxoacid Acids, Bases & Salts Dimensionless An acid in which the ionisable hydrogen is attached to oxygen, which in turn is bonded to a central atom. HₙXOₘ

pH Acids, Bases & Salts Dimensionless The negative logarithm to base ten of the hydrogen ion concentration, measuring the acidity of a solution. pH = −log[H⁺]

pH Scale Acids, Bases & Salts Dimensionless The conventional scale, usually running from 0 to 14, on which the acidity or alkalinity of aqueous solutions is expressed. 0 (acidic) – 7 (neutral) – 14 (alkaline)

Phenolphthalein Acids, Bases & Salts Dimensionless An acid–base indicator that is colourless in acid and pink in alkaline solution, changing between pH 8.3 and 10.0. C₂₀H₁₄O₄

pKa Acids, Bases & Salts Dimensionless The negative logarithm of the acid dissociation constant, giving a convenient scale of acid strength. pKa = −log Ka

pKb Acids, Bases & Salts Dimensionless The negative logarithm of the base dissociation constant. pKb = −log Kb

pOH Acids, Bases & Salts Dimensionless The negative logarithm of the hydroxide ion concentration. pOH = −log[OH⁻]; pH + pOH = 14

Polyprotic Acid Acids, Bases & Salts mol/L An acid capable of donating more than one proton per molecule. H₂A ⇌ H⁺ + HA⁻ ⇌ 2H⁺ + A²⁻

Proton Affinity Acids, Bases & Salts kJ/mol The energy released when a gaseous species accepts a proton, measuring intrinsic basicity free of solvent effects. B(g) + H⁺(g) → BH⁺(g)