Pauling's rules for oxoacid strength

Faced with dozens of oxoacids of wildly different strengths, you might expect a complicated theory. Linus Pauling noticed something delightfully simple instead: write any oxoacid as (HO)pEOq, count just the terminal oxygens q (the ones NOT carrying an H), and that number alone roughly predicts its strength. Two short rules capture nearly the whole p-block.

Rule one: for an acid (HO)pEOq, the first acid dissociation constant Ka1 climbs steeply with q, the number of terminal (non-OH) oxygens. As a rough guide, pKa1 is about 8 minus 5q. So q=0 (like HOCl, hypochlorous acid) gives a very weak acid, pKa near 7-8; q=1 (HNO2, H2SO3) gives moderate acids, pKa near 2-3; q=2 (HNO3, H2SO4 first proton) gives strong acids, pKa near minus 3; q=3 (HClO4) gives a super-strong acid. The reason is electronic: each extra terminal O pulls electron density off the central atom and, in the conjugate base, lets the negative charge delocalize over more oxygens, stabilizing it and so favoring proton loss. Rule two: for a polyprotic acid, each successive proton is harder to remove, with pKa rising by about 5 each step (the second proton leaves a more negative ion, which clings to it harder).

These rules are estimates, not exact laws — Pauling was capturing a real electronic trend with a back-of-envelope formula, and exceptions exist (carbonic acid looks anomalous partly because most dissolved CO2 is never hydrated to H2CO3 at all). But for ranking unfamiliar oxoacids, deciding whether something is weak or strong, or sanity-checking a pKa, counting terminal oxygens is astonishingly powerful. It is one of the cleanest predictive shortcuts in descriptive inorganic chemistry.