This paper presents a semi-theoretical computer model that estimates individual and mean ionic activity coefficients in the NaCl-NaOH-HCl-H₂O system at 25°C. This extra thermodynamic model incorporates long-range electrostatic ion-ion interaction (Debye- Hückel effects), short-range ion-ion interactions (ion association), and short-range ion-solvent interactions (hydration). The activity of water in NaCl, NaOH, and HCl solutions is fit with maximum deviations from experimental values of 0.78%, 0.79%, and 2.09%, respectively. Ion size parameters, å, were modified slightly from literature values. Hydration numbers for individual species were chosen on the basis of best fit. Ion pair dissociation constants of 15.0 were chosen for reactions involving NaCl(aq), NaOH(aq), and HCl(aq) ion pairs. The model predicts individual ionic activity coefficients for ions and ion pairs, and predicts mean molal ionic activity coefficients for NaCl, NaOH, and HCl for solutions up to 6.0 m with maximum deviations from experimental values of 0.73%, 1.77%, and 3.86%,respectively. The estimated degree of dissociation varies widely if the ion pair dissociation constants are varied between 5 and 1000. Calculated values for trace activity coefficients, saturation solubility, and vapor pressure compare favorably to experimental data. / Master of Science