Question

Mixing together solutions of acetic acid and sodium hydroxide can make a buffered solution. Explain. How does the amount of each solution add to change the effectiveness of the buffer? Would a buffered solution made by mixing HCl and NaOH be effective? Explain.

Short answer

On adding HCl acid, the added H⁺ ions react with A^- to form the weak acid. Thus, free H⁺ ions do not accumulate.

On adding OH^- ions, they do not allow to accumulate but are replaced by A^-ions. So, no pH change happens.

Step-by-step solution

Major Species Present

The major species present in the solution before any reaction occurs are HC₂H₃O₂, Na⁺, C₂H₃O₂^-, OH^-, and H₂O.

The dominant equilibrium involves the dissociation of acetic acid.

Effect of adding acid or base

When a base is added:

${\mathbf{OH}}^{\mathbf{-}}\mathbf{+}{\mathbf{CH}}_{\mathbf{3}}\mathbf{COOH}\mathbf{\rightleftarrows }{\mathbf{CH}}_{\mathbf{3}}{\mathbf{COO}}^{\mathbf{-}}\mathbf{+}{\mathbf{H}}_{\mathbf{2}}\mathbf{O}$

The net result is that OH^- ions are not allowed to accumulate.

While if an acid is added:

${\mathbf{H}}^{\mathbf{+}}\mathbf{+}{\mathbf{CH}}_{\mathbf{3}}{\mathbf{COO}}^{\mathbf{-}}\mathbf{\rightleftarrows }{\mathbf{CH}}_{\mathbf{3}}\mathbf{COOH}$

The net result is that H⁺ ions are not allowed to accumulate.

Effect of adding NaOH and HCl

If we mix HCl and NaOH, the acid will be neutralized with the base and we will obtain a neutral salt, not a buffer. For a buffer to work, both the acid and the base component must bepart of the same equilibrium system, i.e., they should be conjugate acid and base.