We have predicted the osmotic and activity coefficients of strong electrolyte solutions using a modification of the Pitzer equation. The modified equation can be used for multicomponent aqueous solutions by applying a mixing rule at the Debye-Huckel term. We have found that the modification of the Pitzer equation retains the accuracy of the original equation without using any characteristic parameters evaluated from the experimental data. The new equation is predictive and simpler than the original Pitzer equation.