Question

A buffer is created by combining 150.0 mL of 0.25 M HCHO2 with 75.0 ml of 0.20 M NaOH. Determine the pH of the buffer

Answer 1

Moles HCHO2 = 0.150 L X 0.25 mol/L = 0.0375 moles HCHO2

Moles NaOH = 0.075 L X 0.20 mol/L = 0.015 mol NaOH

When NaOH is added to the solution it reacts stoichometricly with the HCHO2 to form 0.015 moles of CHO2- ion. The rest of the acid (0.03- 0.015) = 0.015 moles remains as the acid.

So, to calculate the pH of the buffer solution, you can use the Henderson-Haselbalch equation:

pH = pKa + log [formate]/[formic acid]

pH = 3.74 + log (0.015)/(0.015) = 3.74

Thank you.