What is the relationship between a weak acid and a buffer? give me an example and explain how a weak acid can act as a buffer?

Explanation:

Buffer solutions are solutions capable of maintaining their pH at approximately constant values when small amounts of acid or base are added.

A buffer solution is characterized by simultaneously containing a weak species and its conjugate pair. In other words, it has a weak acid and a salt of the same acid from a strong base or, a base and a salt of this base from a strong acid.

Two examples are:

Given that the buffer solution is a mixture of weak acid with a salt of the same acid from a strong base, the acid will be partially dissociated according to the equation:

HA ⇔

Applying the Law of Mass Action (which establishes the relationship between the masses of reagents and products in a chemical equilibrium at a given temperature) and taking into account the dissociation constant() is obtained the following expression:

Where pKa represents the potential value of the acid acid constant of the weak acid,   is the  common anion concentration, equivalent to salt and  [HA] indicates weak acid concentration  which is part of the buffer solution. Consequently, the above equation can be rewritten as follows:

According to this equation, the pH of a solution  buffer, depends on two factors:

a) The pKa value of the weak acid

b) The proportions between salt and acid concentrations  

In summary, the presence of the conjugate pair causes the weak species to dissociate to a lesser extent and the presence of the weak species causes the conjugate pair to hydrolyze less.

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the answer for sodium is 0.186

the answer for rubidium is 0.251