How do you know the molecular formula in chemistry?

if it is called the sum formula of e.g. butane, methane, pentane, hexane… you have to learn this by yourself, e.g. Butan C4 H10 or are there possibilities to find out on the basis of the periodic system?

You should know that : for something with 1 carbon atom, ethane represents something with 2 C atoms, butane represents something with 3 carbon atoms, etc. In fact, a bite of memorizing is at stake.

Then, if necessary, you can simply record a chain where correspondingly many carbon atoms hang together… and fill and count all free bonds with H atoms.

Or you will notice the general formula for alkanes, namely C(n)H(2n+2). Ethane then has n=2 C atoms and (2n+2)=6 H atoms.

Then we have as funny tasks as in the picture. How does it work when there are 2 or 3. What do you change in the spelling?

Actually, this is already written as a finished “step-by-step guide”.

If you have the same side chain several times, it is just a “di-” or “tri-” or “tetra-” methyl/ethyl/propyl thing. The “yl” in each case denotes a side chain.

A hexane having two methyl side chains on 2nd and 3rd C would thus be, for example, a 2,3-dimethylhexane.

If there are different side chains, you name them alphabetically. A hexane with a methyl side chain on 2nd C and an ethyl side chain on 3rd C is accordingly a 3-ethyl-2-methylhexane.

What’s the point of the job in the middle?

Lies’ from the back to the front:

• a pentane
• with two methyl groups
• at positions 3 and 3.

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