Calculating the Molar Mass of Polydimethylsiloxane
Molar mass is the mass of one mole (6.022×1023) of particles. To find it for polydimethylsiloxane, the base polymer of silicone rubber, we only need its composition: multiply the molar mass of each element by its count and add the results.
Start with the composition
Polydimethylsiloxane is a long chain built from a repeating unit that contains 2 carbon atoms, 6 hydrogen atoms, 1 oxygen atom and 1 silicon atom, that is (CH3)2SiO, or C2H6OSi. One mole of these units therefore contains 2 mol of carbon, 6 mol of hydrogen, 1 mol of oxygen and 1 mol of silicon.
Add it up
| Element | Moles | Mass |
|---|---|---|
| Carbon | 2 | 12 × 2 = 24 g |
| Hydrogen | 6 | 1.008 × 6 = 6.048 g |
| Oxygen | 1 | 16 × 1 = 16 g |
| Silicon | 1 | 28.09 × 1 = 28.09 g |
The sum is 24 + 6.048 + 16 + 28.09 = 74.138 g, so the molar mass of the repeating unit is about 74.138 g/mol. The earlier version of this article gave the total as 104.138, an addition error that has been corrected here.
The whole polymer chain
The molar mass of a complete chain is roughly the molar mass of the repeating unit multiplied by the degree of polymerization n, plus the mass of the end groups. Grades of silicone rubber with different degrees of polymerization have different molecular weights, usually expressed as an average.