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The experimental determination and modeling of osmotic coefficients in electrolyte solutions requires knowledge of the stoichiometric coefficient ν(i). In contrast to strong electrolytes, weak electrolytes exhibit a concentration-dependent stoichiometric coefficient, which directly influences the thermodynamic properties (e.g., osmotic coefficients). Neglecting this concentration dependence leads to erroneous osmotic coefficients for solutions of weak electrolytes. In this work, the concentration dependence of the stoichiometric coefficients and the influence of concentration on the osmotic coefficient data were accounted for by considering the dissociation equilibria of aqueous sulfuric and phosphoric acid systems. The dissociation equilibrium was combined with the ePC-SAFT equation of state to model osmotic coefficients and densities of electrolyte solutions. Without the introduction of any additional adjustable parameters, the average relative deviation between the modeled and the experimental data decreases from 12.82% to 4.28% (osmotic coefficients) and from 2.59% to 0.89% (densities) for 12 phosphoric and sulfuric systems compared to calculations that do not account for speciation. For easy access to the concentration-dependent stoichiometric coefficient, estimation schemes were formulated for mono-, di-, and triprotic acids and their salts.