The stronger the acid, the easier it is to ionize, which means that it must have a stable conju gate base. Conversely, a weak acid is reluctant to ionize because it has an unstable conjugate base. The other side of this coin is that unstable anions A− make strong bases and their conju gate acids AH are weak acids.

● Acid and conjugate base strength

The stronger the acid HA, the weaker its conjugate base A−.

 The stronger the base A−, the weaker its conjugate acid AH.

For example, hydrogen iodide has a very low pKa, about –10. This means that HI is a strong enough acid to protonate almost anything. Its conjugate base, iodide ion, is therefore not basic at all—it will not deprotonate anything. A very powerful base is methyllithium, MeLi. Although it is actually a covalent compound, as we discuss in Chapter 9, for the purpose of the discussion here you can think of MeLi as CH3 −Li+. CH3 − can accept a proton to become neutral methane, CH4. Methane is therefore the conjugate acid. Clearly, methane isn’t at all acidic—its pKa is estimated to be 48. The table below gives a few inorganic compounds and their approximate pKa values.

The approximate pKa values of some inorganic compounds

Notice that the lower down the periodic table we go, the stronger the acid. Notice also that oxygen acids are stronger than nitrogen acids. We have also put down more exact pKa values for water but you need remember only the approximate values of 0 and 14. Over the next few pages we shall be considering the reasons for these differences in acid strength but we are fi rst going to consider the simple consequences of mixing acids or bases of different strengths. Notice the vast range covered by pKa values: from around –10 for HI to nearly 50 for methane. This corresponds to a difference of 10⁶⁰ in the equilibrium constant.