Relative Formula Mass (Mr)

Definition of Relative Formula Mass (Mr)

Relative Formula Mass, symbolised as Mr, is the sum of the relative atomic masses of all the atoms in a chemical formula of a compound. It tells you the total mass of one formula unit of the compound compared to one twelfth of the mass of a carbon-12 atom.

It is important to note that Mr has no units because it is a ratio of masses, not an absolute mass.

Note: The term formula unit refers to the simplest ratio of ions in an ionic compound, which is what Mr represents for ionic substances. For molecular compounds, the term relative molecular mass is sometimes used instead.

Calculating Relative Formula Mass (Mr)

To calculate the Mr of a compound:

• Identify all the atoms in the chemical formula.
• Find the relative atomic mass (Ar) of each element from the periodic table.
• Multiply the Ar of each element by the number of atoms of that element in the formula.
• Add all these values together to get the total Mr.

Remember to count every atom shown in the formula, including those in brackets and any numbers outside brackets.

For instance, to calculate the Mr of water, H₂O:

Hydrogen (H) has an Ar of 1, and there are 2 hydrogen atoms.

Oxygen (O) has an Ar of 16, and there is 1 oxygen atom.

So, Mr = (2 d 1) + (1 d 16) = 2 + 16 = 18

For example, calculating the Mr of sodium chloride (NaCl) involves adding the Ar of sodium and chlorine: 23 + 35.5 = 58.5.

Use of Relative Formula Mass (Mr) in Chemistry

Relative formula mass is very useful in chemistry because it allows chemists to:

• Compare the masses of different compounds easily.
• Calculate the masses of substances that react together in chemical reactions (reacting masses).
• Provide a foundation for mole calculations, which are essential for understanding how much of each substance is involved in a reaction (see the topic Moles, Mass and Mr for more details).

For example, knowing the Mr of sodium chloride (NaCl) helps to compare its mass with other compounds or to calculate how much sodium chloride is needed to react with a certain amount of another substance.