Isomerism
One formula, every molecule it can describe — drawn, not listed.
Enumerated up to 9 carbons. Past that the count runs into the hundreds and the page stops being something you can look at.
Description of this simulation
C5H12 has 3 structural isomers. They are 2-methylbutane, 2,2-dimethylpropane, pentane. Every one has the same molecular formula and the same molar mass, and they are different substances with different boiling points.
C5H12
3
structural isomers
Counted by generating every skeleton and discarding the duplicates, not read from a table — so every isomer counted is one you can see below.
They boil at different temperatures
- pentane309.2 K
- 2-methylbutane301 K
- 2,2-dimethylpropane282.6 K
Same formula, same molar mass, different substances. The more branched an isomer is, the more nearly spherical it is, the less its molecules touch, and the lower it boils.
Same formula, different molecule
A molecular formula is not an identity. It says which atoms are present and nothing about how they are joined, and the joining is what decides how a substance behaves.
What this model shows — and what it simplifies
The isomers are enumerated rather than looked up: every parent chain and every branch placement is generated, then anything drawn on less than its longest chain, anything that would give a carbon five bonds, and any duplicate of a skeleton already found is discarded. The counts that come out — 1, 1, 1, 2, 3, 5, 9, 18, 35 — are a consequence of the structures shown, not a table sitting beside them.
Where do we see this in real life?
Petrol is graded on this. Straight-chain octane knocks in an engine and its branched isomer does not, and the octane number on the pump is a measure of which isomers are in the tank.