Question

You combine 0.75 moles formate and 0.85 moles formic acid to make a buffer solution. The Ka of formic acid is 1.8x10-4 what is the pH of the solution

Answer 1

pH = 3.68

Step-by-step explanation:

We can solve this problem by using Henderson-Hasselbach's equation:

• pH = pKa + log
  `([Formate])/([Formic Acid])`

• Where pKa = -log(Ka)

• pKa = -log(1.8x10⁻⁴) = 3.74

Assuming we have 1 L of the buffer solution then the molar concentrations of formate and formic acid would be:

• [Formate] = 0.75 mol / 1 L = 0.75 M

• [Formic Acid] = 0.85 mol / 1 L = 0.85 M

We now have all required data to calculate the pH:

• pH = 3.74 + log
  `(0.75)/(0.85)`

• pH = 3.68