of lead combines with oxygen to form
of a lead oxide. Use this information to work out the formula of the lead oxide (Relative atomic masses:
and
).
Lead
Oxygen
The mole ratio of
in the product is 1:2, which means that for every atom of lead, there will be two atoms of oxygen. The formula of the compound is
.
Vinegar, which is used in our homes, is a dilute form of acetic acid. A sample of acetic acid has the following percentage composition: 39,9% carbon, 6,7% hyrogen and 53,4% oxygen.
Determine the empirical formula of acetic acid.
Determine the molecular formula of acetic acid if the molar mass of acetic acid is
.
In
of acetic acid, there is
,
and
Empirical formula is
The molar mass of acetic acid using the empirical formula is
. Therefore the actual number of moles of each element must be double what it is in the empirical formula.
The molecular formula is therefore
or
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.
