Q19.15P - Chemistry: Molecular Nature Of Matter And Change

Question

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

Step-by-Step Solution

Answer

The pH is obtained is 3.89 .

Step 1: Define Ionic Equilibrium

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

Step 2: 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.