Fundamentals of Acids and Bases

Because acetic acid is a weak acid, we can make the assumption that x is much smaller than the initial concentration of HAc.

According to the Pauling-Ricci rule, pK _a = 8 - 9f + 4n , f for HClO₄ is 3 whereas HClO₃ has f = 2. Conceptually we can understand why HClO₄ has the lower pK _a: the central atom Cl becomes more electronegative when it is adjoined to more electron-withdrawing oxygen atoms, and therefore the Cl atom withdraws electron density away from the oxygen holding the acidic proton. This weakens the bond between the oxygen and the proton.

The S-H bond in H₂S is much weaker than the O-H bond in H₂O because O is more electronegative than S. The weakly bonded proton is a mark of a stronger acid.