Calculating pH of a Buffer Solution
Calculate the pH of the solution
A 1.49 L buffer solution consists of 0.264 M propanoic acid and 0.149 M sodium propanoate. Calculate the pH of the solution following the addition of 0.073 mol HCl. Assume that any contribution of the HCl to the volume of the solution is negligible. The Ka of propanoic acid is 1.34 × 10-5.
Question
What is the pH of the solution after adding HCl?
Answer:
pH = 4.37
Explanation:
In 1.49L of buffer solution you will contain:
1.49L × (0.264mol Propanoic acid / 1L) = 0.393mol propanoic acid
1.49L × (0.149mol Sodium propanoate / 1L) = 0.222mol Sodium propanoate
Reaction of HCl with sodium propanoate is:
HCl + sodium propanoate → Propanoic acid + water
That means in the reaction you consume sodium propanoate producing propanoic acid doing moles of each compound are:
0.393mol propanoic acid + 0.073mol = 0.466 mol propanoic acid
0.222mol Sodium propanoate - 0.073mol = 0.149 mol Sodium propanoate
Using Henderson-Hasselbalch formula for propanoic acid / sodium propanoate buffer:
pH = pka + log₁₀ [Sodium propanoate] / [ Propanoic acid]
pH = pka + log₁₀0.149 mol /0.466 mol
pKa for this buffer is -log1.34x10⁻⁵ = 4.87
Replacing:
pH = 4.87 + log₁₀0.149 mol /0.466 mol
pH = 4.37
I hope it helps!