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The acidic strength of the acid present in the solution is measured by the acid dissociation constant$\left(K_a\right)$ . It is the dissociation equilibrium constant for acidic reactions.

The acidic reaction is represented by $K_a$. Trichloroacetic acid $\left({\mathrm{Cl}}_3{\mathrm{CCO}}_2\mathrm{H}\right)$dissociates to the trichloroacetate ion $\left({\mathrm{CI}}_3{\mathrm{CCO}}_2^{-}\right)$ and the proton $\left({\mathrm{H}}^{+}\right)$. Consider the following $K_a$reaction:

${\mathrm{ClCCO}}_2{\mathrm{H}}^{+}→{\mathrm{ClCCO}}_2^{-}+{\mathrm{H}}^2$

The acidic reaction is represented bylocalid="1663391982837" $K_a$.Because the anilinium ion is an unstable species, it dissociates into aniline and proton. Consider the followinglocalid="1663392020349" $K_a$reaction:

Now, the $K_a$represents the acidic reaction. Lanthanum ion separates into lanthanum hydroxide$\left({\mathrm{LaOH}}^{2+}\right)$and proton$\left({\mathrm{H}}^{+}\right)$. The$K_a$reaction is as follows:${\mathrm{La}}^{3+}{\mathrm{H}}_2\mathrm{O}={\mathrm{LaOH}}^{2+}+{\mathrm{H}}^{+}$