is a product of a quotient of concentrations [ H + ] [ A − − ] [ HA ] and a quotient, Γ Γ , of activity coefficients γ γ H + γ γ A − − γ γ H A . In these expressions, the quantities in square brackets signify the concentration of the undissociated acid, HA, of the hydrogen ion H, and of the anion A ; the quantities γ γ are the corresponding activity coefficients. If the quotient of activity coefficients can be assumed to be a constant which is independent of concentrations and pH, the dissociation constant, K a can be expressed as a quotient of concentrations.