The hydrogen ion concentration in weak acid of dissociation constant Ka and concentration c is nearly equal to

HA   ⇄  H3O+ + A-At equilibrium      C1-α    Cα           CαH3O+ = Cα   and Ka=Cα×CαC(1-α)=Cα2 ⇒ α=Kac  So H3O+ = C  Kac = Kac