Admin P: 22,712 $$\frac {[HA]}{[A^-]} = 10^{pK_a-pH}$$ Works for every acid, always. In words - when you are 1 pH unit below pKa ratio of concentrations of HA/A- is 10, when you are 2 pH units below ratio is 100, when pH is 1 unit above pKa ratio is 0.1 and so on. Makes wonders when you try to estimate what ions are present in the acid solution of known pH.