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Strong and Weak Acids and Bases

The most common strong acids and bases, and some examples of weak acids and bases, together with definition of strong and weak acids and bases.

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A strong acid  or a strong base completely ionizes (dissociates) in a solution. In water, one mole of a strong acid HA dissolves yielding one mole of H+ (as hydronium ion H3O+) and one mole of the conjugate base, A.  Essentially, none of the non-ionized acid HA remains.

Strong acid:                   HA + H2O  →  A-(aq)  +  H3O+(aq)

Strong base:                 BOH + H2O  →  B+(aq)  +  OH-(aq)

Examples of strong acids and bases are given in the table below.  In aqueous solution, each of these essentially ionizes 100%.

A weak acid or a weak base only partially dissociates. At equilibrium, both the acid and the conjugate base are present in solution

Weak acid:                 AH  + H2O  ↔   A-(aq)  +  H3O+(aq)

   Weak base:              BOH  +   H2O   ↔    B+(aq)   +   OH-(aq)   or

                                    B  +  H2O   ↔    BH+(aq)   +  OH-(aq)

Examples of weak acids and bases are given in the table below.

Stronger acids have a larger acid dissociation constant (Ka) and a smaller logarithmic constant (pKa = −log Ka) than weaker acids. The stronger an acid is, the more easily it loses a proton, H+.

Two key factors that contribute to the ease of deprotonation are the polarity of the H—A bond and the size of atom A, which determines the strength of the H—A bond. Acid strengths also depend on the stability of the conjugate base.

See also Acid-base properties of aqueous solutions of salts with ions from both acids and bases, Buffer solutionspKa of inorganic acids and bases, pKa of phenols, alcohols and carboxylic acids and pKa of amines, diamines and cyclic organic nitrogen compounds, as well as Acid and base pH indicators or pH values of some solutions of acids and bases.

Stong Acids
Strong Bases
Hydrobromic acid HBr Barium hydroxide Ba(OH)2
Hydrochloric acid HCl Calsium hydroxide Ca(OH)2
Hydroiodic acid HI Lithium hydroxide LiOH
Nitric acid HNO3 Potassium hydroxide KOH
Perchloric acid HClO4 Sodium hydroxide NaOH
Sulfuric acid H2SO4 Strontium hydroxide Sr(OH)2

Weak acids
           Weak bases
Acetic acid CH3COOH Ammonia NH3
Carbonic acid H2CO3 Diethylamine (CH3CH2)2NH
Formic acid CHOOH Methylamine CH3NH2
Hydrocyanic acid HCN Sodium bicarbonate NaHCO3
Hydrofluoric acid HF
Phosphoric acid H3PO4
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