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The relative atomic mass, Ar, of an elementis the average mass of the naturally occurring isotopes of the element relative to the mass of an atom of carbon-12.
The relative molecular mass, Mr, is the mass of a molecule relative to the mass of an atom of carbon-12, which is given the exact mass of 12.
Example: NH3 = 14 + 3x1 = 17.
The relative formula mass is used for compounds made up of ions.
A mole of substance is the amount of substance that has the same number of particles as there are in 12.00g of carbon-12. The particles may be atoms, molecules, ions or even electrons.
This number of particles is a constant known as Avagadros constant (L), and is approximately 6 x 1023mol-1.
The mass of one mole of a substance is often referred to as the molar mass (M). The units of molar mass are gmol-1.
To find the amount of substance present in a given mass, we must divide that mass by the molar mass of the substance.
For example - if we had 10g of CaCO3:
M is 40 + 12 + 3x16 = 100g.
So in 10g of calcium carbonate there is 10/100 mol = 0.1 mol CaCO3.
To find the mass of a given substance, we multiply the number of moles of the substance by the molar mass.
If we know the mass of a reactant, we can find the mass of a product in a chemical equation.
Consider the formation of ammonia from hydrogen and nitrogen.
2N2 (56g) + 3H2 (6g) → 4NH3 (34g)
Hence, for every 6g of hydrogen we make 34g of ammonia.