Empirical formula

The molecular formula of an alkane shows the number of carbon and hydrogen atoms in each molecule. It shows the ratio of carbon atoms (C) to hydrogen atoms (H).

For example the formula of ethane is C2H6. The ratio of C:H atoms is 2:6.

An empirical formula shows the simplest whole number ratio of atoms of each element present in a substance.

The empirical formula of an alkane is based on the simplest ratio of carbon to hydrogen atoms:

  • the simplest form of the C:H ratio for ethane is 1:3
  • the empirical formula is CH3 (note that the 1 is not written)
Question

What is the empirical formula of butane, C4H10?

The C:H ratio for butane is 4:10

The simplest ratio is 2:5

So the empirical formula is C2H5

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