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Relative Atomic Mass (Ar)
The Relative Atomic Mass, Ar, is the weighted average mass of an atom of an element, taking
into account its naturally occurring isotopes, relative to 1/12 of an atom of Carbon12.
Ar = Average mass of one atom of an element × 12
Mass of one atom of Carbon12.
Relative Molecular Mass (Mr)
The relative molecular mass, Mr, of a molecule is the mass of that molecule compared to 1/12 the
relative atomic mass of an atom of Carbon12.
The Relative Molecular Mass can be found by adding up all the Relative Atomic Masses of all
atoms present in the molecule. (See Table)
Molecule Formula Ar of Atoms Mr
Water H2O (2 × 1.0) + 16.0 18.0
Carbon Dioxide CO2 12.0 + (2 × 16.0) 44.0
Methane CH4 12.0 + (4 × 1.0) 16.0
The Avagadro Constant
The number of atoms in 12g of Carbon12.
6.022 × 1023
The Mole is the amount of substance that contains 6.022 × 1023 particles.
The Mole of a substance is the Ar in grams.
Number of Moles = Mass in grams
Mass of one Mole in grams
How many moles of atoms are there in 64.2g of Sulphur, S?
Ar of Sulphur = 32.1, therefore 1 mole of Sulphur is 32.1g.
Number of Moles = 64.2 = 2.00 Moles
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The Empirical formula represents the simplest ratio of the atoms of each element present in a
Steps to work out the Empirical formula of a substance:
A sample of a hydrated compound was analysed and found to contain 2.10g of
Cobalt. 1.14g of Sulphur. 2.28g of Oxygen, and 4.50g of Water. Calculate its Empirical
Component Co S O H2O
Mass / Grams 2.10 1.14 2.28 4.50
n = / m 2.10 = 0.0356 1.14 = 0.0355 2.28 = 0.…read more
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So, the molecular formula is the same as the empirical formula. C2H6O
The Ideal Gas Equation
If we take one mole of gas, the constant is given the symbol R and is called `the gas constant'.
For n moles of gas:
Notes on Units...…read more