How is the relative molecular mass used in empirical formula calculations?

The relative molecular mass is used in empirical formula calculations to determine the ratio of atoms in a compound.

In more detail, the empirical formula of a compound represents the simplest whole-number ratio of atoms of each element present. To calculate this, you need to know the relative amounts of each element, which is where the relative molecular mass comes in.

The relative molecular mass (Mr) is the sum of the relative atomic masses of the atoms in a molecule. It is a dimensionless quantity, meaning it has no units. It is simply a comparison of the mass of one molecule to the mass of another.

To use the relative molecular mass in empirical formula calculations, you first need to know the percentage composition of the compound. This is the percentage by mass of each element in the compound. You can then convert these percentages into moles by dividing by the relative atomic mass of each element.

Once you have the number of moles of each element, you can then find the simplest whole-number ratio. This is done by dividing each number of moles by the smallest number of moles calculated. This will give you the ratio of atoms, which is the empirical formula.

For example, if you have a compound with a relative molecular mass of 180 and it is made up of 40% carbon, 6.7% hydrogen and 53.3% oxygen, you would first convert these percentages into moles. This would give you 3.33 moles of carbon, 6.67 moles of hydrogen and 3.33 moles of oxygen. Dividing each of these by the smallest number of moles (3.33) gives you the empirical formula of CH2O.

So, the relative molecular mass is a crucial piece of information when calculating the empirical formula of a compound. It allows you to determine the ratio of atoms, which is fundamental to understanding the composition of the compound.