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Strong acids completely dissociate into ions in the solvent. This process releases proton and the solution is termed acidic. Conversely, the weak acids don't dissociate completely into individual ions in solution. Thus, the solution of weak acid has both dissociated and non-dissociated forms of the acid existing in equilibrium. The dissociation constant of weak acids is represented by ${\mathrm{K}}_{\mathrm{a}}$.

The ${\mathrm{K}}_{\mathrm{a}}$value actually measures the strength of the acid - how readily it could release a proton or how readily a substance exhibits its acidic behaviour. The weak acids have a lower ${\mathrm{K}}_{\mathrm{a}}$value indicating that they don't lose proton easily and that they are not completely dissociated. But indicating the acidic strength of weak acids merely by ${\mathrm{K}}_{\mathrm{a}}$is not appropriate and applicable all the time that the value corresponding to the negative logarithm of ${\mathrm{K}}_{\mathrm{a}}$value is considered as the expression for the acidity of weak acids. Mathematically we could write,

${\mathrm{pK}}_{\mathrm{a}}=-{\log }_{10}{\mathrm{K}}_{\mathrm{a}}$

${\mathrm{K}}_{\mathrm{a}}$and ${\mathrm{pK}}_{\mathrm{a}}$values are inversely proportional to each other. Weaker acids having lower${\mathrm{K}}_{\mathrm{a}}$value have higher${\mathrm{pK}}_{\mathrm{a}}$value. Stronger acids having higher ${\mathrm{K}}_{\mathrm{a}}$value will have lower${\mathrm{pK}}_{\mathrm{a}}$value.

Stronger acids have higher ${\mathrm{K}}_{\mathrm{a}}$value and lower${\mathrm{pK}}_{\mathrm{a}}$value. Lower${\mathrm{pK}}_{\mathrm{a}}$value means the value can be less than zero also. Very strong acids such as${\mathrm{HCIO}}_4,{\mathrm{CF}}_3{\mathrm{SO}}_3\mathrm{H},{\mathrm{CH}}_3{\mathrm{SO}}_3\mathrm{H},\mathrm{HNO}$have higher${\mathrm{K}}_{\mathrm{a}}$value and their${\mathrm{pK}}_{\mathrm{a}}$values are less than zero - that is they have negative${\mathrm{pK}}_{\mathrm{a}}$value. Chemical formula of compound with negative${\mathrm{pK}}_{\mathrm{a}}$was written.