Buffer Capacity Calculator
Capacity peaks at the pKa and falls away fast — a buffer two units off is barely a buffer.
The formula
pH = pKa + log10([A-] / [HA])
Why equal amounts land on the pKa
When the conjugate base and the weak acid are present in equal concentration their ratio is one, the logarithm of one is zero, and the pH sits exactly at the pKa. The concentrations themselves drop out entirely: a 1 M and a 0.01 M acetate buffer mixed one-to-one both read 4.76.
What the concentrations do change
They set the capacity. A dilute buffer holds the same pH but surrenders it after a few drops of acid; a concentrated one absorbs far more before moving. Capacity peaks at the pKa and falls away quickly on either side, which is what fixes the working range at roughly one pH unit each way.
Which denominator?
Most of the confusion in solution work is not arithmetic but bookkeeping. Molarity divides by the volume of the finished solution; molality divides by the mass of the solvent; percent by mass divides by the mass of the solution again. The three agree closely in dilute water and diverge sharply in concentrated acid, which is why a bottle labelled 98 % is also labelled 18.4 M without contradiction.