Ka acid dissociation constant

Consider the dissociation of a weak acid HA.
HA(aq) ≒ H^+(aq) + A^-(aq) .. Ka

Ka = [H^+][A^-]/[HA]
-log(Ka) = -log([H^+][A^-]/[HA])
-log(Ka) = -log[H^+] - log([A^-]/[HA])
pKa = pH - log[A^-] + log[HA]
pKa - pH = log[HA] - log[A^-]
pH - pKa = log[A^-] - log[HA]

Refer to the above mathematical formula.

When pH > pKa of the acid:
pH - pKa > 0
log[A^-] - log[HA] > 0
log[A^-] > log[HA]
[A^-] > [HA]
(Concentration of anionic form A^-) > (Concentration of neutral form HA)
Hence, the acid will mainly exist as the anionic form A^-.

When pH < pKa of the acid:
pH - pKa < 0
log[A^-] - log[HA] < 0
log[A^-] < log[HA]
[A^-] < [HA]
(Concentration of anionic form A^-) < (Concentration of neutral form HA)
Hence, the acid will mainly exist as the neutral form HA.