Henderson-Hasselbalch equation
/ HEN-der-sun HAS-ul-bahlk ee-KWAY-zhun /
Suppose you want a solution at a particular pH and you have a weak acid and its salt on the shelf. Instead of guessing and re-measuring, you would like a recipe that says how much of each to mix. The Henderson-Hasselbalch equation is that recipe, written as a single short line.
It states that the pH of a buffer equals the pKa of the weak acid plus the base-10 logarithm of the ratio of the conjugate base concentration to the weak acid concentration. In plain terms: start from the acid's pKa, then nudge up or down depending on whether the base or the acid form dominates. When the two forms are present in equal amounts, the logarithm is zero and the pH simply equals the pKa.
The equation matters because it turns buffer-making into arithmetic and shows at a glance why buffers work best near their pKa, where small ratio changes barely move the pH. The honest caveat is that it is an approximation: it uses concentrations instead of activities and assumes the acid and base amounts you mixed are essentially unchanged by dissociation, so it strays at very low concentration, at extreme pH, or in salty solutions.
To make a buffer at pH 4.76 you mix equal moles of acetic acid (pKa 4.76) and sodium acetate; the logarithm of one is zero, so the equation predicts pH equal to the pKa, and a pH meter confirms it.
Equal acid and base forms make pH equal the pKa exactly.
The ratio in the equation is base over acid, not acid over base. Flipping it inverts the correction and pushes the predicted pH the wrong way, a classic exam-time mistake.