The pK a of a molecule, or the negative logarithm of the acid-base dissociation constant, is related to the equilibrium constant for the dissociation of a particular acid into its conjugate base and a free proton. The pK a also corresponds to the pH at which the corresponding acid and its conjugate base each are populated equally in solution. Given that the pK a corresponds to a transition between specific protonation states, a given molecule may have multiple pK a values.