Q19.8P - Chemistry: Molecular Nature Of Matter And Change

Question

A chemist needs a pH 3.5 buffer. 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?

Step-by-Step Solution

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 1: 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.

Step 2: 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 of the 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.