Question

What is the relationship between the strength of an acid and its pK value?

Short answer

The${\mathrm{pK}}_{\mathrm{a}}$ and the strength of the acid is inversely proportional to each other.

Step-by-step solution

Acid strength

The acid strength is indicated by the${\mathrm{pK}}_{\mathrm{a}}$ value. The acid dissociation constant, or ${\mathrm{pK}}_{\mathrm{a}}$, is the negative log of the${\mathrm{K}}_{\mathrm{a}}$ value.

Relationship of the pKa value to the acidic strength

The pH scale classifies values below 7 as acidic and those over 7 as basic, whereas, pH 7 is consideredneutral or physiological pH. The amount of ${\mathrm{H}}^{+}$ions in solution increases under an acidic state, lowering the pH. Similarly, an increase in the concentration of the${\mathrm{OH}}^{-}$ (hydroxyl) ion elevates the pH value over 7.

Dissociation constant of Ka

The acidic strength will be proportionate to the value of ${\mathrm{K}}_{\mathrm{a}}$, i.e. the value of${\mathrm{k}}_{\mathrm{a}}$ is directly related to the acidity.

$\begin{array}{l}\mathrm{pH}=-\log \left({\mathrm{H}}^{+}\right)\\ {\mathrm{pK}}_{\mathrm{a}}=-\log \left({\mathrm{K}}_{\mathrm{a}}\right)\end{array}$

Conclusion

Therefore, thehigher the${\mathrm{K}}_{\mathrm{a}}$ value, the lower will be the value ${\mathrm{PK}}_{\mathrm{a}}$. Thelower the${\mathrm{PK}}_{\mathrm{a}}$ value, thestronger will be acid.