We can use the acidity constant, Ka, to calculate the H3O+ concentration. Since the Ka is a dissociation constant for the reaction HA -> A- + H+ we can surmise that the concnetration of the conjugate base, A- and the hydronium ion, H+ will be equal. This normally would lead to the need to solve a quadratic equation to determine those concentrations, but it turns out that we can use a simplifying approximation most of the time so that the quadratic formula is seldom necessary in these calculations. This procedure is described for acids and for weak bases as well in the following video.
