Acids, Bases & Salts

Diprotic Acid

An acid that can donate two protons per molecule.

H₂A ⇌ 2H⁺ + A²⁻ mol/L

Double Salt

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 Dimensionless

Acids, Bases & Salts

End Point

The point in a titration at which the indicator changes colour, signalling that the reaction is judged complete.

mL

Acids, Bases & Salts

Equivalence Point

The point in a titration at which exactly stoichiometric amounts of the two reactants have been combined.

n_acid × basicity = n_base × acidity mol

Acids, Bases & Salts

Henderson–Hasselbalch Equation

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]) Dimensionless

Acids, Bases & Salts

Hydrolysis Constant

The equilibrium constant for the hydrolysis of a salt ion in water.

Kh = Kw / Ka (anion) mol/L

Acids, Bases & Salts

Indicator Range

The span of pH over which an indicator's colour visibly changes, roughly one unit either side of its pKa.

pH = pK_In ± 1 Dimensionless

Acids, Bases & Salts

Ionic Product of Water

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 mol²/L²

Acids, Bases & Salts

Levelling Effect

The tendency of a solvent to reduce all acids or bases stronger than its own ionised forms to the same effective strength.

Dimensionless

Acids, Bases & Salts

Lewis Theory

The theory defining an acid as an electron pair acceptor and a base as an electron pair donor.

A + :B → A←B Dimensionless

Acids, Bases & Salts

Methyl Orange

An acid–base indicator that is red in acid and yellow in alkali, changing between pH 3.1 and 4.4.

C₁₄H₁₄N₃NaO₃S Dimensionless

Acids, Bases & Salts

Monoprotic Acid

An acid that can donate only one proton per molecule.

HA ⇌ H⁺ + A⁻ mol/L

Acids, Bases & Salts

Neutral Salt

A salt whose aqueous solution has a pH of 7 because neither of its ions hydrolyses appreciably.

Dimensionless

Acids, Bases & Salts

Neutral Solution

A solution in which the concentrations of hydrogen and hydroxide ions are exactly equal.

[H⁺] = [OH⁻] = √Kw mol/L

Acids, Bases & Salts

Ostwald's Dilution Law

The relation showing how the degree of ionisation of a weak electrolyte increases on dilution.

Ka = cα² / (1 − α) mol/L

Acids, Bases & Salts

Oxoacid

An acid in which the ionisable hydrogen is attached to oxygen, which in turn is bonded to a central atom.

HₙXOₘ Dimensionless

Acids, Bases & Salts

pH

The negative logarithm to base ten of the hydrogen ion concentration, measuring the acidity of a solution.

pH = −log[H⁺] Dimensionless

Acids, Bases & Salts

pH Scale

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) Dimensionless

Acids, Bases & Salts

Phenolphthalein

An acid–base indicator that is colourless in acid and pink in alkaline solution, changing between pH 8.3 and 10.0.

C₂₀H₁₄O₄ Dimensionless

Acids, Bases & Salts

pKa

The negative logarithm of the acid dissociation constant, giving a convenient scale of acid strength.

pKa = −log Ka Dimensionless

Acids, Bases & Salts

pKb

The negative logarithm of the base dissociation constant.

pKb = −log Kb Dimensionless

Acids, Bases & Salts

pOH

The negative logarithm of the hydroxide ion concentration.

pOH = −log[OH⁻]; pH + pOH = 14 Dimensionless

Acids, Bases & Salts

Polyprotic Acid

An acid capable of donating more than one proton per molecule.

H₂A ⇌ H⁺ + HA⁻ ⇌ 2H⁺ + A²⁻ mol/L

Acids, Bases & Salts

Proton Affinity

The energy released when a gaseous species accepts a proton, measuring intrinsic basicity free of solvent effects.

B(g) + H⁺(g) → BH⁺(g) kJ/mol