Acidity constant Guide, Meaning , Facts, Information and Description

In chemistry and biochemistry, the acidity constant or acid dissocation constant () is a specific type of dissociation constant that indicates the extent of dissociation of hydrogen ions from an acid. An acid with an acidity constant near 1 is almost completely dissociated when dissolved in water; conversely, an acid with an acidity constant near 0 remains almost completely undissociated. In lay terms, the higher the , the stronger the acid in question.

Because this number varies over many degrees of magnitude, the acidity constant is often represented by the inverse of its common logarithm, represented by . (cf. pH).

Given a weak acid HA, its dissolution into water is subject to the following equilibrium:

HA + H₂O ↔ H₃O⁺ + A^–

This is often written as:

HA ↔ H⁺ + A^–

The species A^– is referred to as the conjugate base of the acid. The acidity constant for the acid HA is the dissociation constant for this equilibrium. Thus:

where [X] denotes the molar concentration of X in the solution.

By analogy, one can define the basicity constant (, and similarly ) of the conjugate base A^–:

For the equilibrium:

A^– + H₂O ↔ HA + OH^–

Analogously to , the magnitude of indicates the relative strength of the base, with closer to 1 indicating a much stronger base.

The relation between K_a and K_b is:

where K_w is the dissociation constant of water, which is 1.0x10^-14 mol² dm^-6at 20 °C.

As the product of K_a and K_b must remain a constant, it follows that stronger acids will have weaker conjugate bases, and vice versa.

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