An appreciable number of HB molecules exist in water solution. An acid-base reaction normally goes in the direction of the weaker acid. You can use this fact to compare the relative strengths of any two acids. If we have 0.10 M of the following acids, we obtain the following results. This is known as the leveling effect of water.
Strong acids form conjugate bases which have no measurable strength in water.
Within a series of acids, the stronger the acid, the weaker the conjugate base, and the reverse is also true. The weaker the acid the stronger the conjugate base. The strength of the HX bond is important. The stronger the HX bond the less easily the substance will be ionized, the weaker the acid. In going down a column of elements of the periodic table, as the size of atom X increases the HX bond strength decreases and the strength of the binary acid increases.