How do I calculate pH from pKa and concentration?

The pH of a solution can be calculated if the pKa of the acid and the concentrations of the acid (HA) and its conjugate base (A⁻) are known. This is done through using the Henderson-Hasselbach equation: pH = pKa + log₁₀([HA]/[A⁻]). This equation shows that the pH of the solution depends on the pKa of the acid and the ratio of the concentrations of the acid to its conjugate base. The Henderson-Hasselbach equation is a useful tool for estimating pH in many cases, but it's not accurate for concentrated solutions or extreme pH values such as very low pH acids or very high pH bases.