Question

Find the pH of a buffer that consists of 0.45M HCOOH and 0.63M HCOONa ( ${\mathbf{\text{pK}}}_{\mathbf{\text{a}}}$of HCOOH=3.74)?

Short answer

The pH is obtained is 3.89 .

Step-by-step solution

Define Ionic Equilibrium

AN ionic equilibrium refers to an equilibrium that exists in the solution of weak electrolytes between unionized molecules and ions.

The pH of the buffer solution

Write the 0.45 M HCOOH dissociation equation first.

${\text{HCOOH +H}}_{\text{2}}\text{O}\rightleftarrows {\text{HCOO}}^{\text{-}}{\text{+H}}_{\text{3}}{\text{O}}^{\text{+}}.$

Then, write the 0.63 M HCOONa dissociation equation.

$\text{HCOONa}\to {\text{HCOO}}^{\text{-}}{\text{+ Na}}^{\text{+}}.$

We now have the initial formic acid and formate concentrations.

$$\begin{gathered} \text{HCOOH = 0.45 M} \\ {\text{HCOO}}^{\text{-}}\text{= 0.63 M} \end{gathered}$$

To find the pH, we may utilize the Henderson-Hasselbalch equation.

$$\begin{gathered} \text{pH = pKa + log}\frac{\left[{\text{HCOO}}^{\text{-}}\right]}{\left[\text{HCOOH}\right]} \\ \text{= 3.74 + log}\frac{\text{0.63 M}}{\text{0.45 M}} \\ \text{= 3.89.} \end{gathered}$$

Therefore, the pH value is3.89.