# How to Calculate pH and pKa of a Buffer using Henderson-Hasselbalch Equation?

Henderson-Hasselbalch equation is a numerical expression which relates the pH, pKa and Buffer Action of a buffer. A buffer is a solution which can resist the change in pH. Chemically, a buffer is a solution of equimolar concentration of a weak acid (such as acetic acid – CH3COOH) and its conjugate base (such as acetate ion – CH3COO¯). In the previous post, we have discussed the Titration Curve of a weak acid and the Derivation of Henderson-Hasselbalch Equation. The characteristic shape of the titration curve of a weak acid is also described by the Henderson-Hasselbalch equation. In this chapter we will discuss the methods to calculate the pH or pKa of a buffer using Henderson-Hasselbalch equation using sample problems.

Henderson-Hasselbalch Equation is given as:

Where,

pH – the negative logarithm of H⁺ ion concentration in the medium.

pKa – the negative logarithm of Ka of the acid (Ka is the dissociation constant)

Proton acceptor – the ionized or deprotonated acid (example – CH3COO¯).

Proton donor – intact (non-ionized) weak acid (example – CH3COOH).

Let’s see some sample problems and solutions.

Problem-1: A mixture of 0.20M acetic acid and 0.30M sodium acetate is given. Calculate the pH of the medium if the pKa of the acetic acid is 4.76.