If the molar volumes of solute species are known, the density of a solution can be calculated as explained in aqueous molar volumes and density of solutions.
The figure compares the calculated density of evaporating seawater with measured data from Baseggio (1974) and Brantley et al. (1984), cited by Chambers et al. (1993). The agreement is quite good upto halite saturation and perhaps beyond (PHREEQC input file dens.phr).
The calculated electrical conductivity of the evaporated solution is plotted on the secondary Y-axis. The calculated EC is accurate initially, but may be less so when the SO4-2-concentration increases above 0.1 mol/L where gypsum starts to precipitate (see PHREEQC's calculation of the specific conductance).
Chambers, L.A., Wadsley, M.W. and Brereton, G.J., 1993. Modelling halite formation and brine densities: comparison of non-marine and seawater brines. Seventh Symp. Salt, 1, 533-538, Elsevier.