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
- E.g.: CO2
- Ar of C=12
- Ar of O=6 there are two oxygen atoms in the compound so 6x2
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- 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.
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