Which expression is the Henderson and Hasselbalch equation?

1. Start from the acid constant and make the hydronium concentration the subject. 2. That gives hydronium equal to the constant times acid over conjugate base. 3. Take the negative logarithm of both sides of that relation. 4. The ratio flips when the sign changes, giving pH equal to pKa plus log of base over acid. 5. When the two components are equal the logarithm term is zero and the pH equals the pKa. _Source: OpenStax Chemistry (CC BY 4.0), Ch 14 "Acid-Base Equilibria", section 14.6 Buffers_