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Relative Molecular Mass (Mr)
Mr is calculated by adding together all the
relative atomic masses (Ar) in a molecule/formula
For example
NaCl - 1 sodium + 1 Chlorine
= (1 x 23) + (1 x 35.5)
= 58.5
ZnCl2 ­ 1 Zinc + 2 Chlorine
= (1 x 65) + (2 x 35.5)
= 136…read more

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Number of Moles
Number of moles = mass of X
of substance X Ar or Mr of X
For example
7g of Nitrogen (Ar 14) = 7/14 = 0.5 moles
4g of silicon (Ar 28) = 4/28 = 0.14 moles
10g of CaO (Mr 56) = 10/56 = 0.18 moles…read more

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Percentage Composition
To calculate the percentage of one element in
a compound
% composition = Ar of X x 100
of X in compound Mr of compound
For example
% of Na (Ar 23) in NaCl (Mr 58.5)
= (23/58.5) x 100 = 39%…read more

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Maximum Yield
Maximum yield is the maximum mass of a
substance that could be produced in a chemical
reaction
X+Y XY
Maximum yield = (mass of X) x (Mr of XY)
Ar of X
For example ­ what is the maximum mass of zinc
oxide (Mr 81) when 20g of Zn (Ar 65) reacts?
maximum yield = (20/65) x 81 = 24.9g…read more

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Percentage Yield
Percentage yield is how much you actually
get compared to what is theoretically possible
(maximum yield)
% yield of X = mass of X obtained x 100
maximum yield
For example ­ what is the percentage yield if
you get 14 g of ZnO when the maximum yield
is 24.9g?
Maximum yield = (14/24.9) x 100 = 56%…read more

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