How can I estimate the volume of a molecule?

I would like to know the size of one molecule of methyl orange?
Udit Saxena 3/16/99

Vocabulary
density*
methyl orange
molar volume*
molecular weight*
If the volume of empty space in a solid is negligible compared to the volume occupied by molecules, a quick and dirty estimate of the molecular volume can be obtained by dividing the volume of one mole of the material by Avogadro's number. To compute the molar volume, you need the material's density and its molecular weight. For methyl orange, the density of the solid is approximately 1.00 g/mL, and the molecular weight is 327.34, so the molar volume is

(327.34 g/mol) × (1 mL/1.00 g) = 327 mL/mol

and the average volume per molecule is

327 mL/mol × (1 mol/6.02×1023 molecules) = 5.43 × 10-22 mL/molecule

These aren't really actual molecular volumes; they're the average contribution each molecule makes to the total volume of the crystal. When the molecules are packed in a regular way (as they are in crystals), these average molecular volumes are easy to relate to the average center-to-center distance between molecules. The relationships are sometimes used to explain volume changes that accompany phase changes.

Author: Fred Senese senese@antoine.frostburg.edu



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