What is the difference between relative formula mass and relative molecular mass?
They are often taken to be the same but there is a technical difference. The relative molecular mass, as its name implies, strictly only applies to substances which contain molecules, i.e. small groups of atoms connected by covalent bonds. The relative formula mass applies to all substances, whatever their structure and relates to the formula of the substance as it is written, e.g. NaCl, H2O2, O2, Xe, etc. If you stick to relative formula mass, the concept of relative molecular mass becomes redundant.
