Editing Acid (section)

==Acid strength==
{{main|Acid strength}}

The strength of an acid refers to its ability or tendency to lose a proton. A strong acid is one that completely dissociates in water; in other words, one [[mole (unit)|mole]] of a strong acid HA dissolves in water yielding one mole of H<sup>+</sup> and one mole of the conjugate base, A<sup>−</sup>, and none of the protonated acid HA. In contrast, a weak acid only partially dissociates and at equilibrium both the acid and the conjugate base are in solution. Examples of [[strong acid]]s are [[hydrochloric acid]] (HCl), [[hydroiodic acid]] (HI), [[hydrobromic acid]] (HBr), [[perchloric acid]] (HClO<sub>4</sub>), [[nitric acid]] (HNO<sub>3</sub>) and [[sulfuric acid]] (H<sub>2</sub>SO<sub>4</sub>). In water each of these essentially ionizes 100%. The stronger an acid is, the more easily it loses a proton, H<sup>+</sup>. Two key factors that contribute to the ease of deprotonation are the [[chemical polarity|polarity]] of the H—A bond and the size of atom A, which determines the strength of the H—A bond. Acid strengths are also often discussed in terms of the stability of the conjugate base.

Stronger acids have a larger [[acid dissociation constant]], ''K''<sub>a</sub> and a lower p''K''<sub>a</sub> than weaker acids.

[[Sulfonic acid]]s, which are organic oxyacids, are a class of strong acids. A common example is [[toluenesulfonic acid]] (tosylic acid). Unlike sulfuric acid itself, sulfonic acids can be solids. In fact, [[polystyrene]] functionalized into polystyrene sulfonate is a solid strongly acidic plastic that is filterable.

[[Superacid]]s are acids stronger than 100% sulfuric acid. Examples of superacids are [[fluoroantimonic acid]], [[magic acid]] and [[perchloric acid]]. The strongest known acid is [[helium hydride ion]],<ref name="chebi33689">{{cite web |title=Hydridohelium (CHEBI:33689) |url=https://www.ebi.ac.uk/chebi/searchId.do?chebiId=CHEBI%3A33689 |work=Chemical Entities of Biological Interest (ChEBI) |publisher=European Bioinformatics Institute}}</ref> with a [[proton affinity]] of 177.8kJ/mol.<ref name="Epa">{{cite journal |last1=Lias |first1=S. G. |last2=Liebman |first2=J. F. |last3=Levin |first3=R. D. |year=1984 |title=Evaluated Gas Phase Basicities and Proton Affinities of Molecules; Heats of Formation of Protonated Molecules |journal=Journal of Physical and Chemical Reference Data |volume=13 |issue=3 |pages=695 |bibcode=1984JPCRD..13..695L |doi=10.1063/1.555719}}</ref> Superacids can permanently protonate water to give ionic, crystalline [[hydronium]] "salts". They can also quantitatively stabilize [[carbocation]]s.

While ''K''<sub>a</sub> measures the strength of an acid compound, the strength of an aqueous acid solution is measured by pH, which is an indication of the concentration of hydronium in the solution.  The pH of a simple solution of an acid compound in water is determined by the dilution of the compound and the compound's ''K''<sub>a</sub>.