Question

A chemist needs a pH3.5buffer. Should she use NaOH with formic acid (${\mathbf{\text{K}}}_{\mathbf{\text{a}}}{\mathbf{\text{= 1.8×10}}}^{\mathbf{-}\mathbf{4}}$) or with acetic acid (${\mathbf{\text{K}}}_{\mathbf{\text{a}}}{\mathbf{\text{= 1.8×10}}}^{\mathbf{-}\mathbf{5}}$)? Why? What is the disadvantage of choosing the other acid? What is the role of the NaOH?

Short answer

The chemist should use a formic acid since its ${\text{pK}}_{\text{a}}$is closer to the required pH. As a buffer, an acetic acid alone will not be sufficient. A strong base, such as NaOH, can help maintain the optimal pH by neutralizing the ${\text{H}}_{\text{3}}{\text{O}}^{\text{+}}$ generated by the acid.

Step-by-step solution

Preparation of the buffer with required pH value

To prepare an acidic buffer solution with the required pH value, the acid is chosen in such a way that its pKa value is nearer to the required pH value.

Role of NaOH

First, solve for the pKa value of the acids.

The pKa value of the formic acid is calculated as:

$$\begin{gathered} {\text{pK}}_{\text{a}}\text{= - log}\left({\text{K}}_{\text{a}}\right) \\ {\text{= - log(1.8×10}}^{-4}\text{)} \\ =3.74 \end{gathered}$$

The pKa value ofthe Acetic acid is calculated as:

$$\begin{gathered} {\text{pK}}_{\text{a}}\text{= - log}\left({\text{K}}_{\text{a}}\right) \\ {\text{= - log(1.8×10}}^{-5}\text{)} \\ =4.74 \end{gathered}$$

A buffer with a pH of 3.5 is required by the chemist. The chemist should choose an acid with a ${\text{pK}}_{\text{a}}$value that is closer to the buffer pH. As a result, the chemist must utilize the formic acid. Because the pH variation will be more than 1 , using an acetic acid will not generate an effective buffer. The buffer component ratio must exceed the effective range, which is 0.1 to 10 . The purpose of NaOH in the buffer is to neutralize the ${\text{H}}_{\text{3}}{\text{O}}^{\text{+}}$created by the acid, allowing us to change the pH efficiently. This was included to keep a track of the intended pH.

Therefore, since a formic acid has a ${\text{pK}}_{\text{a}}$value that is closer to the target pH, it should be used. An acetic acid alone will not suffice as a buffer. By neutralizing the ${\text{H}}_{\text{3}}{\text{O}}^{\text{+}}$created by the acid, a strong basic such as NaOH can aid in maintaining the optimum pH.