Molecular size is a measure of the area a molecule occupies in three-dimensional space. The amount of space any mass takes up in three-dimensional space is known specifically as its volume. Using algebra and the density formula discovered by Archimedes of Syracuse, one can determine the molecular size of a molecule for any given mass of molecular substance.

Let density equal mass over volume. (p (Greek letter rho) = m / v)

Plug in the molecular values for the density equation.

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Multiple both sides of the equation by v over 1. This removes the fraction and results in the equation v x p = m.

Divide both sides by p (rho). (vp= m is equal to v = m / p). The resulting value for v is the volume or three-dimensional size of the molecule.