pKa
/ pee-kay-AY /
Acid strengths span an absurd range — some acids are billions of times stronger than others — and raw Ka values like 0.000018 are awkward to compare. pKa is the tidy fix: it squeezes that whole range onto a friendly little scale of small numbers, the same trick pH uses for acidity.
Formally, pKa is the negative base-10 logarithm of the acid dissociation constant: pKa = -log(Ka). Because of the minus sign, the relationship flips: a small pKa means a large Ka, hence a strong acid, while a large pKa means a weak one. A change of one in pKa corresponds to a tenfold change in Ka, so pKa values that differ by a few points describe acids of vastly different strength.
pKa matters because it is the single most-quoted number for comparing acids, choosing buffers, and predicting which way a proton will jump. The simple rule of thumb: a proton tends to move from the acid with the lower pKa to form the species with the higher pKa. Just remember the inverse logic — lower pKa means stronger, which trips up newcomers expecting bigger to mean stronger.
Hydrochloric acid has a pKa around -7 (very strong), acetic acid about 4.76 (weak), and water about 15.7 (barely acidic). The lower the pKa, the more eagerly the proton departs.
Lower pKa = stronger acid; each unit of pKa is a tenfold step in strength.
There is a handy fact: when a solution's pH equals the acid's pKa, the acid and its conjugate base are present in exactly equal amounts. That is why a buffer is most effective right at its pKa.