The composite density, or solution density, refers to the total density of a chemical mixture or composition of materials. Density is the mass of a substance per unit volume such as pounds per gallon. Every substance has an individual density which is dependent upon temperature. If multiple substances are mixed into a homogeneous solution, the composite or solution density can be found by multiplying each substance's density by its solution concentration.
Things You'll Need
 Calculator

Determine the substances (species) to be mixed into a solution. For instance, 2 lb. of species A is added to 3 lb. of species B and 4 lb. of species C. The total solution mass is 9 lb. and the mass concentration (mass fraction) of each substance is 0.22 (A), 0.33 (B) and .045 (C). The sum of the mass fractions should always equal 1.0.

Determine the density of each component in the solution. For instance, species A has a density of 7.5 lb. per gallon, species B has a density of 8.3 lb. per gallon and species C has a density of 9.1 lb. per gallon.

Determine the solution density using the mass fractions and densities of each component. This is done with the equation (density A x 0.22) + (density B x 0.33) + (density C x 0.45). The answer is 8.484 lb. per gallon.
References
 Photo Credit Thomas Northcut/Lifesize/Getty Images