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K_a (acidity constant or acid-ionization constant) is a quantitative measure of an acid's strength in solution. In the context of acid-base processes, it is the equilibrium constant for a chemical reaction known as dissociation.

K_a is the constant of dissociation of acid as

\begin{aligned}{HA(aq)\to{H^+}(aq)+{A^-}(aq)}\\{{K_a}=\frac{{c\left({{H^+}}\right)\times c\left({{A^-}}\right)}}{{c(HA)}}}\end{aligned}

For stronger acids,K_a has greater dissociation. A strong acid has a stable conjugated base and does not react with conjugated acid to produce the reactant. A weak acid does not have dissociated conjugate base, is not stable, and reacts with conjugated acid to produce the reactant. Therefore, from the same initial concentration of weak and strong acids, strong acids have more protons. pH is lower in strong acids and higher in weak acids.

HI has largerK_a, it is a stronger acid than HF×I^- is a more stable conjugated base than F^-, as it has a larger atomic radius.