where wj and νj are the mass fraction and the specific volume of component j, and the sum is over all the components in the solution . Equation 1 is readily derived from the assumption that the total volume of the solution is the sum of the volumes of the individual molecules; see Supplemental Information (SI) A. The weighting factor is w since specific volume is a massic quantity (volume/mass); it would be mole fraction x for the corresponding molar quantity molar volume (volume/mole). Equation can be re-written for the mass density of the mixture ρmix as