The Henderson Hasselbach equation

We can write a generalized equation for the dissociation of an acid as follows:

(1)

Where A- is the anion of the acid HA. The dissociation constant as derived by the law of mass action is given by the equation

(2)

This can be rewritten using the respective logarithms as

(3)

(2) may then be rearranged taking the log of both sides

(4)

Which may be written as

(5)

If we substitute pH for -log [H+] we have the following general equation (the Henderson-Haselbach equation). This allows us to calculate pH of an acid base pair if we know the pK'.

(6)