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The residual part of acid after losing a proton (H⁺) will have a tendency to accept a proton (H⁺). Therefore, it will behave as a base. These pairs of substances which differ from one another by a proton(H⁺) are known as conjugate acid-base pairs. Consider a general example of an acid:

A single structural formula sometimes cannot explain all the properties of a compound that is given. In such cases, the compound may be represented by two or more structural formulae which differ from each other only in the arrangement of electrons. None of this structural formula alone can explain all the observed properties of the compound. The compound is then said to show resonance. The various structures are known as resonating structures. The true structure of the molecule is not represented by any of the resonating structures but is considered to be a resonance hybrid of the various resonating structures. Resonance delocalization is considered as a dominating effect in stabilizing the conjugate base.

K_ais known as the acid dissociation constant and its value indicates the relative strength of the acid. pK_ai s defined as the negative logarithm (base ) of K_a. Mathematically, pK_a.= -logK_a value of strong acids is usually zero or even negative while pK_athe value of weaker acids are greater than 4.

Each compound reacts with a generic base to give the conjugate base.

With the addition of nitro group, the pK_avalue decreases drastically. The acidity increases with nitro group because the anions (conjugate bases) that are formed are more stabilized by delocalization and also, they have more resonance structures.