What is Ka in chemistry?

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

An acid, ##HA##, undergoes the following equilibrium in water:

##HA(aq) + H_2O(l) rightleftharpoons H_3O^+ + 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 equation).

As for any equilibrium, we can write equality of forward and reverse rates as:

##K_a## ##=## ##([H_3O^+][A^-])/([HA(aq)])##

##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.

##pK_a## values, where ##pK_a=-log_10K_a##, are another of this approach.