Chemical Quantities: Relative Formula Masses

The relative formula mass (Mr) of a compound is calculated by adding up the relative atomic masses (Ar) of all the atoms in its formula.
Relative atomic masses are found on the periodic table. Each element’s entry includes its Ar.
The relative formula mass of a molecule is the sum of the relative atomic masses of the atoms in the numbers shown in the formula.
For example, the Mr of water (H2O) is 18 [2(1) + 16]. This is calculated by adding the Ar of two hydrogen atoms (1 each) to the Ar of one oxygen atom (16).
The units for relative formula mass are technically “relative mass units,” but these are often left off in calculations and discussions.
Relative formula mass is a unitless quantity. This is because it is a ratio of masses, compared to the mass of a carbon-12 atom.
Calculations involving relative formula mass can be used to predict the masses of reactants and products in chemical reactions (stoichiometry).
It’s important to remember that in chemical reactions involving gases, volume ratios can be used instead of mass ratios. This reflects the fact that equal volumes of different gases contain equal numbers of molecules (Avogadro’s Law).
The concept of moles is closely tied to relative formula mass. One mole of a substance contains 6.022 x 10^23 particles of that substance (a number known as Avogadro’s number), and has a mass equal to the Mr of that substance in grams.
For example, one mole of water (H2O) has a mass of 18 grams.
This relationship allows for the conversion between moles, mass, and numbers of atoms/molecules in chemical quantities problems.