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Determining Empirical Formulae of Simple
Compounds from Reacting Masses
There are a large number of simple compounds - each with its own
empirical formula. However, the formulae are all calculated by a
similar method:…read more

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Determining Empirical Formulae
An empirical formula is found by carrying out an experiment to
calculate the different proportions of the elements taking part in
a reaction.
In this way, the formula of a product can be calculated from the
masses of the reactants.
The element carbon-12 atom is used as the standard relative to
which all other atomic masses are measured.
Each atom has a RAM (Ar ) that describes how many times
heavier it is than the mass of 1/12 of a 12C atom.
Relative atomic mass (Ar) 24
Atomic Number 12…read more

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Determining Empirical Formulae
In an experiment, it was found that 7g of nitrogen reacted with 16g
of oxygen. Use this information to calculate the empirical formula
of the compound formed. (Ar nitrogen = 14 Ar oxygen = 16).
Create a table as shown below
Divide the reacting mass by its Ar
Calculate the ratio of atoms to each other
Nitrogen Oxygen
Ar = Ar =
14 16
7 / 14 = 0.5 16 / 16 = 1.0
1 2
The ratio of oxygen to nitrogen atoms is 2:1
- so the formula for this oxide is NO2…read more

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Calculating The Formula of Magnesium Bromide
Carry out the following empirical formula calculation for yourself:
A student carried out an experiment to determine the formula of
magnesium bromide. The student found that 0.12g of magnesium
reacted with 0.8g of bromine. Use this information to work out the
empirical formula of magnesium bromide.
(Ar magnesium = 24 Ar bromine =
Magnesium Bromine
Ar = 24 Ar = 80
0.12 / 24 = 0.005 0.8 / 80 = 0.01
1 2
The ratio of magnesium atoms to bromine atoms is 1:2
- so the formula for magnesium bromide is MgBr2…read more


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