What is the pH of a solution of 0.15 M formic acid?

Calculate the pH of a buffer solution that is 0.250M in formic acid, HCOOH, and 0.200 M in formate ion, HCOO-.?

• Calculate the pH of a buffer solution that is 0.250M in formic acid, HCOOH, and 0.200 M in formate ion, HCOO-. The ionization constant for formic acid is 1.80^

• Answer:

  You can use either the expression for Ka, or you can calculate pKa and use the Henderson-Haselbalch equation. For the first method: Ka = 1.8 X 10^-4 = [H+][Formate]/[formic acid] 1.8 X 10^-4 = [H+](0.200)/0.250) [H+] = 2.25 X 10^-4 pH = 3.65 Using the Henderson-Haselbalch equation: pH = pKa + log [formate]/[formic acid] pH = 3.74 + log (.200)/(0.250) = 3.64 The difference between the two is just a rounding issue, and not a big deal