Write the Henderson-Hasselbalch equation for a propanoic acid solution (CH3CH2CO2H, pKa = 4.874) using the symbols HA and A-, and the given pKa value for propanoic acid in the expression

The Henderson-Hasselbalch approximation is for conjugate acid-base pairs in a buffered solution. We're going to call HA a weak acid, and A - its conjugate base. The equation is as follows:

pH = pKa + log ([base]/[acid]), where the brackets imply concentrations

Plugging in our symbols and the pKa value, the equation becomes:

pH = 4.874 + log ([A-]/[HA])