this is wrong the answer is 2.1 because x is small and doesn\'t contribute to the over molar equation. so x of H+ ends up being = to Ka. -log of ka = 2.1. also, when X- is = HX, pH = pKa Solution this is wrong the answer is 2.1 because x is small and doesn\'t contribute to the over molar equation. so x of H+ ends up being = to Ka. -log of ka = 2.1. also, when X- is = HX, pH = pKa.