# What is Ka in chemistry?

Sep 28, 2016

This is the acid dissociation constant, which quantifies the extent of acid dissociation in a solvent, typically water.

#### Explanation:

An acid, $H A$, undergoes the following equilibrium in water:

$H A \left(a q\right) + {H}_{2} O \left(l\right) r i g h t \le f t h a r p \infty n s {H}_{3} {O}^{+} + {A}^{-}$

For strong acids, this equilibrium lies strongly to the right; and for weak acids, the equilibrium lies to the left (as written, i.e. as we face the scree).

As for any equilibrium, we can write equality of forward and reverse rates, i.e. the condition of equilibrium, as:

${K}_{a}$ $=$ $\frac{\left[{H}_{3} {O}^{+}\right] \left[{A}^{-}\right]}{\left[H A \left(a q\right)\right]}$

${K}_{a}$ is thus simply a number that must be measured for different acids at different temperatures; there is wide disparity in ${K}_{a}$ values. For strong acids, ${K}_{a}$ values are large (typically greater than 10); for weaker acids, ${K}_{a}$ are much smaller, and the equilibrium equation lies to the left.

$p {K}_{a}$ values, where $p {K}_{a} = - {\log}_{10} {K}_{a}$, are another refinement to this approach. The stronger acids typically have NEGATIVE $p {K}_{a}$ values...i.e. the extent of protonolysis is generally considered to be quantitative