An investigation into the water of crystallisation present in Hydrated Magnesium Sulphate

Aim

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

Hypothesis

I predict that if I take a mass of approximately 2.46g of hydrated magnesium sulphate 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:

Equipment

Apparatus

Bunsen burner;

Heatproof mat;

Tripod;

Pipe clay Triangle;

10 g Hydrated Magnesium sulphate;

Crystallising dish;

Sensitive scales;

Tongs;

Scoop;

Stopwatch;

Gas supply;

Goggles.

Diagram

Method

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.

Fair Testing

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Fair Testing

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 used.

Safety

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.

Experimental Report

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

Analysis

MgSO4.xH2O reference table

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 substance.

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 environment.

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.

Evaluation

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.