Ka and Kb values measure how well an acid or base dissociates. Higher values of Ka or Kb mean higher strength. General Ka expressions take the form Ka = [H3O+][A-] / [HA]. General Kb expressions take the form Kb = [BH+][OH-] / [B]. When using Ka or Kb expressions to solve for an unknown, make sure to write out the dissociation equation, or the dissociation expression, first.