An Investigation Into the Water of Crystallisation Present in Hydrated Magnesium Sulphate
The aim of this experiment is to investigate the amount of water of
crystallisation is present in the hydrous powder, and to investigate
whether or nor the substance is MgSO4.7H2O
I predict that if I take a mass of approximately 2.46g of hydrated
and decompose it by heating until all of the water
of crystallisation has been driven off, I should be left with a mass
of approximately 1.2g anhydrous Magnesium Sulphate.
This size of the starting mass was chosen because it is both practical
to the experiment, due to limited time and supplies, but large enough
to create accurate readings on a 3 decimal placed set of scales.
Calculating the molecular masses of the anhydrous substance created
the figures 246 and 120. These calculations follow:
Pipe clay Triangle;
10 g Hydrated Magnesium sulphate;
Firstly, a crystallising dish's mass will be measured on the scales,
and will be recorded. Next, approximately 2.46g of Hydrated Magnesium
sulphate will be placed into the Crystallising Dish using the scoop.
The mass of the crystallising dish with the Hydrated Magnesium
sulphate in will now be measured and recorded. The mass of the
crucible dish on its own will be subtracted from the mass of the
crystallising dish with the Hydrated Magnesium sulphate in, which will
give the mass of the Hydrated Magnesium sulphate on its own. The
apparatus will then be set up as shown in the diagram, and the
Hydrated Magnesium sulphate will be heated for 3 minutes before its
mass is measured. This will evaporate all the H2O out of the
substance. After this mass has been recorded, the substance will be
replaced over the flame of the Bunsen burner. After one minute the
mass will be re-measured. This will be repeated every minute until
there is no change in the mass. This experiment will then be repeated.
To make this a fair test, it is important that all the variables are
kept constant apart from the heating of the substance to evaporate
water. Also before measuring the mass of any of the objects, it is
important to check the scales are re-calibrated to 0.000 or as near to
as possible in order to ensure there are minimal errors in the
experimental stage. A crucial part of the experiment is to ensure that
the mass crystallising dish and substance is measured a sufficient
amount of times to ensure that all the H2O has been removed from the
Hydrated Magnesium sulphate; otherwise this could affect the analysis
stage of the experiment. Ensuring that no other chemicals or materials
mix with the substance means that each measurement will be as accurate
as experimentally possible. Repeating the experiment is a way of
minimising any errors that occur. The same chemicals must therefore be
Because a Bunsen is being used with the tripod likely to get very hot
it is important to stand at the desk rather than to sit in case
anything falls of the desk that would otherwise land in somebody's
lap. It is also important to wear goggles in case any hot substances
spark off the crystallising dish.
An unknown labelling error caused the two substances used in the
experiment to differ. This did not change the procedure at all, but it
did mean that the average result would simply lie between the two
substances rather than be a more accurate result. However, it did soon
become clear that heating the substance and fully removing all water
was going to take more than 5 minutes, so this period of the
experiment was extended to 15 minutes when it was repeated.
Table of Results
Total mass (g)
Mass of Crucible (g)
Mass of Magnesium (g)
Mass change (g)
% Mass change (to 3 S.F.)
MgSO4.xH2O reference table
Mr (RMM) of hydrated molecule
Mr of the water
% of mass that is water
Predicted % that would remain
The results show, when compared to the reference table, that in the
first experiment, the x in MgSO4.xH2O was above seven, and therefore
above the options handed to the scientists by the governing body. As
there was an unknown labelling area, it will be assumed that this was
it. The results from experiment one will therefore be examined no
further. However, in the second experiment, far more likely results
were achieved. The percentage mass change came out to be 42.3% (to 3
S.F.). This is reasonably close to the 42.85714% of water of he
Pie chart comparing Substance X to MgSO4.5H2O
The pie chart shows that Substance X and MgSO4.5H2O are very similar.
Had they been exactly the same the pie chart would have been split
exactly in two, but one part is slightly larger than the other. But
this is still as accurate as can be expected in a laboratory
The actual difference between substance X and MgSO4.5H2O is only
0.55714%. It is therefore with relative certainty that it can be
stated that substance X actually was MgSO4.5H2O, and not MgSO4.7H2O.
The reason this can be stated is that heating the Hydrated Magnesium
sulphate until the mass no longer lowered ensured all the water had
been evaporated. By using the reference table, which calculates
Relative Molecular Mass' and accurately calculates the percentage of
those molecules that is water.
This does not support my prediction. I predicted that there would be a
mass change of 51.21951%, therefore proving that it was MgSO4.7H2O.
However, this was not the case. But this can only be argued to an
extent due to the failure of the first experiment, it cannot be
determined whether the second experiments findings were anomalous nor
could an average be drawn.
If this experiment were to be repeated, there could be a number of
amendments to improve the accuracy of results. Firstly, it would have
been repeated three times to allow a further error check in case one
of the experiments again went wrong. The experiments would also have
used the exact same substance allowing averages and anomalous results
to be identified, allowing more accurate results to be produced.
However, the experiment did produce a result, that the second
substance used was MgSO4.5H2O. This is what is suggested by the data,
but the data is not very reliable due to there being no way to check
the data to see if it was accurate as two completely different
chemicals were used. Had the experiment and one chemical was used the
results could be declared more accurate and more reliable.
This experiment provided a very accurate result, as it fit nearly
perfectly with the reference table, but there was no way to prove this
was not just co-incidence as the first results were completely
unreliable and unusable. But due to the presence of one result that
could be backed up by the scientific evidence of relative molecular
mass', it is fair to call this experiment a success.