Aluminium trichloride (
) is an ionic substance that forms crystals in the solid phase. Water molecules may be trapped inside the crystal lattice. We represent this as:
. A learner heated some aluminium trichloride crystals until all the water had evaporated and found that the mass after heating was
. The mass before heating was
. What is the number of moles of water molecules in the aluminium trichloride?
We first need to find n, the number of water molecules that are present in the crystal. To do this we first note that the mass of water lost is
.
The next step is to work out the mass ratio of aluminium trichloride to water and the mole ratio. The mass ratio is:
To work out the mole ratio we divide the mass ratio by the molecular mass of each species:
Next we do the following:
and
So the mole ratio of aluminium trichloride to water is:
And now we know that there are 6 moles of water molecules in the crystal.
Khan academy video on molecular and empirical formulae - 1
Khan academy video on mass composition - 1
Moles and empirical formulae
Calcium chloride is produced as the product of a chemical reaction.
What is the formula of calcium chloride?
What percentage does each of the elements contribute to the mass of a molecule of calcium chloride?
If the sample contains
of calcium chloride, what is the mass of calcium in the sample?
How many moles of calcium chloride are in the sample?
of zinc combines with
of sulphur. What is the empirical formula of zinc sulphide?
What mass of zinc sulphide will be produced?
What percentage does each of the elements in zinc sulphide contribute to its mass?
Determine the formula of zinc sulphide.
A calcium mineral consisted of 29,4% calcium, 23,5% sulphur and 47,1% oxygen by mass. Calculate the empirical formula of the mineral.
A chlorinated hydrocarbon compound was analysed and found to consist of 24,24% carbon, 4,04% hydrogen and 71,72% chlorine. From another experiment the molecular mass was found to be
. Deduce the empirical and molecular formula.
Molar volumes of gases
It is possible to calculate the volume of one mole of gas at STP using what we know about gases.
Write down the ideal gas equation
, therefore
Record the values that you know, making sure that they are in SI units You know that the gas is under STP conditions. These are as follows:
Substitute these values into the original equation.
Calculate the volume of
of gas under these conditions The volume of
of gas at STP is
.
The standard units used for this equation are
in
,
in
and
in
. Remember also that
and
.
A sample of gas occupies a volume of
, has a temperature of
and has a pressure of
. Calculate the number of moles of gas that are present in the sample.
The only value that is not in SI units is volume.
.
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