Reacting Masses

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Reacting Masses
It is possible to use moles=mass/mr to deduce the masses of
products and reactants that will react with each other.
Two main points:
The total combined mass of the reactants must be the same as the
total combined mass of the products. This is known as the law of
conservation of mass.
The ratio in which species react corresponds to the number of
moles, and not their mass. Masses must therefore all be converted
into moles, then compared to each other, then converted back.
Reactions which go to completion
What mass of aluminium will be needed to react with 10 g of CuO,
and what mass of Al2O3 will be produced?
3CuO(s) + 2Al(s) Al2O3(s) + 3Cu(s)
10 g
= 10/79.5
= 0.126 moles of CuO
3:2 ratio with Al
so 2/3 x 0.126 = 0.0839 moles of Al, so mass of Al = 0.0839 x 27 = 2.3
3:1 ratio with Al2O3
so 1/3 x 0.126 = 0.0419 moles of Al2O3, so mass of Al2O3 = 0.0419 x
102 = 4.3 g

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