01Buffer pH in one line
pH = pKa + log([A⁻]/[HA]). Mole ratio works too since volume cancels.
Ratio 10:1 → pH = pKa + 1; 1:10 → pKa − 1.
02Notebook box
Use moles after any reaction with strong acid/base.
Worked example: pKa 4.74, 0.20 mol A⁻, 0.10 mol HA → pH = 4.74 + log 2 = 5.04.