# AQA GCSE Chemistry C2 3.2 Masses of atoms and moles

revision notes

HideShow resource information

## Relative atomic mass & Formula mass

• The actual mass of an atom is so small it wouldn't be useful to use in experiments/calculations, so instead the relative mass is used; (Ar)
• E.g.: We use the atom of carbon-12 as a standard for carbon-it gives a mass of exactly 6 protons and 6 neutrons, so it has a 'mass' of 12 units. This is used to compare to other relative atomic masses of other atoms.
• The relative atomic mass of an element is usually the same/similar to the mass number of its most common isotopes of the element which is found naturally-an average mass.
• Relative formula masses (Mr) of compounds:
• E.g.: Sodium Chloride; NaCl.
• Ar of Na=23
• Ar of Cl=35.5
• Mr=23+35.5=58.5
• E.g.: CO2
• Ar of C=12
• Ar of O=6 there are two oxygen atoms in the compound so 6x2
• Mr=12+6x2=24
1 of 2

## Moles

• This is sort of a shorthand of 'relative atomic mass in grams'.
• For example, in an Oxgen atom, with a relative atomic mass of 6, the relative atomic mass in grams has exactly one mole of oxgen atoms.
• A mole is equal to 6.02 x 10^23.
• This means that in 6g of oxygen atoms, there are 6.02 x 10^23 oxygen atoms.
• The same way, the relative atomic mass for carbon in grams is 12g, which means there is a mole of carbon atoms in 12g of it.
2 of 2

Report

omg i love doctor who, sherlock and merlin too! and writing

great resource too

and i hope you get your book published, a friend of mine did

good luck

Report

Haha, looks like we could get along real well anum :D thanks, I'll try!

## Similar Chemistry resources:

See all Chemistry resources »See all Calculations, moles and yield resources »