The pK_a of a Bronsted acid provides a description of the tendency of the acid to donate a proton in an aqueous (water-based) solution. A lower pK_a is associated with a stronger tendency to donate protons, and thus a stronger Bronsted acid. The pK_a is calculated mathematically as the negative logarithm (base 10) of the equilibrium constant for dissociation of a proton from the acid in water.