Question

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?

Answer 1

Step-by-step 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:

• Mixture of acetic acid and sodium acetate
• Ammonium Hydroxide and Ammonium Chloride

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 ⇔
`H^(+)  + A^(-)`

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(
`Ka=([A^(-)]*[H^(+)] )/([HA])`) is obtained the following expression:

`pH=pKa+log([A^(-) ])/([HA])`

Where pKa represents the potential value of the acid acid constant of the weak acid,
`[A^(-) ]` 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:

`pH=pKa+log(salt)/(acid)`

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.