# What is the rule of 13?

##### 1 Answer

The rule of 13 states that the formula of a compound is a multiple

#### Explanation:

The formula is used in interpreting mass spectra.

If you have heteroatoms, you adjust the formula. For example:

- For
#"O"# , add#"O"# and subtract#"CH"_4# . - For
#"N"# , add#"N"# and subtract#"CH"_2# . - For
#""^35"Cl"# , add#"Cl"# and subtract#"C"_2"H"_11"# .

For example, benzamide has a molecular ion at 121, which is an odd number.

The rule of 13 predicts a formula of

The odd mass and the odd number of

This gives a formula

The peak at 77 suggests

We know this contains

The infrared spectrum may give further information.

If it shows a strong peak at

This shows the presence of an O atom.

The formula becomes

The two absorptions in the region

The structure becomes