How does relative molecular mass differ from molar mass?

Relative molecular mass and molar mass are essentially the same, but they are used in different contexts.

Relative molecular mass (Mr) is a dimensionless quantity that tells us how many times heavier a molecule is compared to one-twelfth (1/12th) of the mass of a carbon-12 atom. It is used when we are dealing with individual molecules. For example, the relative molecular mass of water (H2O) is approximately 18 because it is made up of two hydrogen atoms (each with a relative atomic mass of 1) and one oxygen atom (with a relative atomic mass of 16).

On the other hand, molar mass is the mass of one mole of a substance. It is expressed in grams per mole (g/mol). The molar mass of a substance is numerically equal to the relative molecular mass but has the unit of mass. For instance, the molar mass of water is also approximately 18 g/mol. This is because one mole of water molecules has a mass of about 18 grams.

The key difference between the two lies in their application. Relative molecular mass is used when we are considering individual molecules, while molar mass is used when we are dealing with amounts of substance in moles. This distinction is important in chemistry because it helps us to understand and calculate quantities in chemical reactions. For example, knowing the molar mass of a substance can help us to work out how much of it we need to use in a reaction, while the relative molecular mass can help us to understand the proportions in which different substances react.