IA Lab report solubility

Solubility

A comparison of three substances in group 17 of the periodic table

Erik Thörnblad, IB06A-PDP

2007-04-21

Aim

The aim for this lab was to investigate how the altering of anions does affect the solubility of three different substances.

Variables

The independent variables were the salts, which were altered throughout the experiment. The solubility was the dependent variable, and the controlled variables were the temperature and the volume o the water.

Equipment

6.41 g Potassium Chloride,
6.41 g Potassium Bromide,
6.41 g Potassium Iodide,
5 g water, (for each experiment)

Magnetic stirrer, magnet, funnel, filter (0.87 g), two beakers (20 ml 17.27 g), spoon, scale with an uncertainty of ± 0.005 g and a graduated cylinder with an uncertainty of ± 0.05 ml

Method

At first, 5 ml (5 g) water was filled up with the graduated cylinder, and then moved to the beaker. The beaker was then filled with an excess of Potassium Chloride. The beaker was put on a magnetic stirrer, and with a magnet we stirred the solution for two minutes. The solution was then filtrated with the funnel and the filter, and thus we got the excess salt, that had not dissolved in the water, alone in the filter. As the filter became wet, drying was required to evaporate the water. The excess Potassium Chloride was then weighed and compared with the original weight of the Potassium Chloride to find out how much Potassium Chloride that ended up in the solution. The solubility could then be calculated by knowing the mass of the solute and of the solvent. This procedure was repeated with the other salts involved in the experiment.

Result

To calculate the values needed, a scale with an uncertainty of ± 0.005 was used. To calculate our values, the following model was utilized:

5 g water was used in every test, and to get the solubility to be per 100 g water, multiplication with 20 was forced. The uncertainty of g salt in the beaker is ± 0.01, as two uncertainties, each of ± 0.005 were subtracted. The uncertainty of g salt in the filter is ± 0.01, as two uncertainties, each of ± 0.005 were subtracted. The uncertainty of g salt in the solution is ± 0.02, as two uncertainties, ...

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Result

To calculate the values needed, a scale with an uncertainty of ± 0.005 was used. To calculate our values, the following model was utilized:

Using the above model, the following values were found:

Now, the uncertainty of the solubility of KCL is approximately:

The uncertainty of the solubility of KBr is approximately:

The uncertainty of the solubility of KI is approximately:

These three tables with values were then, to more easily see the differences, combined into one:

The values of solubility, presented in g salt that can be dissolved in 100 g water, was then inserted into the below diagram. The solubility among the three salts undoubtedly differs.

Now comparing with the solubility which should have been reached, it is clear that the result reached is not very closed to the result that should have been reached.

Conclusion

We evidentially see that the change of anion does affect the solubility. An equal amount of salt was used in every test, and yet the solubility differed. We had in every test an equal mass of salt, and yet the solubility differed in every test. The trend was that the further you descend in group 17 in the periodic table, the greater the solubility became.

When comparing the results reached with the maximum solubility, we see that complete saturation was for from reached (more about this in the evaluation). For this test, saturation however may not have been necessary as long as one sees a difference in the salt dissolved, which indeed was possible to see.

There are two possible explanations for the difference of solubility among the three substances. What separates the substances are of course their anions (Cl-, Br-, I-). But what quality of the anions is the one affecting the solubility? The first, and perhaps most obvious, would be the weight. If we state the weight of the substance controls the solubility, we can then decide that: the more the anion weighs the greater solubility the solubility will have.

Another possible explanation, linked to the mass of the anions, is the actual amount of the anions. If one investigates the molar masses of Bromide and Chloride ions, one learns that Bromide has a larger molar mass than Chloride. This means that the same mass of the both elements will generate different amounts. In our test, we had the same weight of all three salts, and as the molar masses differ, so did the number of atoms involved in each test. We had a larger number of KCL- than KBr-atoms, which in turn was more than KI-atoms. If one looks on the amount of each substance, one sees that more KI- than KCl-atoms dissolved, whilst KBr dissolved the least. A strange result, which must be blamed on the poor experiment performed. If one looks at the supposed values, one learns that KI should have dissolved the most, followed by KBr and then KCl. Not as our result, then.

The electron arrangement may be another cause to why the solubility differs. In Group 17, where we find our cations, the following can be said about our elements: The further down the group one descends, the closer the electrons are to each other.

Evaluation

One test per salt is far too little. It is not at all certain that the result is reliable. There might have been some kind of error, a human mistake, or just pure chance, which affected the result. It is possible that the results are supposed to be the other way around, with KI as the salt dissolving the least, instead of KCL.

It is not certain whether the test was completely saturated or not. That excess salt actually appeared indicates that the samples were saturated, but still the results are far from plausible. The explanation to this most likely is that a considerable amount of salt stayed in the beaker when the solution was poured into the funnel. Instead of drying the beaker (some water remained) and weighing that salt as well, that salt was simply ignored, despite its importance for the experiment. Ignoring the salt greatly diminishes the trust in this experiment’s results.

Instead of using the graduated cylinder when measuring up the water, the scale should have been used for this as well. The graduated cylinder was only correct to ± 0.05 ml, or to ± 0.05 g (1 ml water = 1 g), whilst the scale had an uncertainty of ± 0.005 g. A more precise result would have been guaranteed using the scale for this as well.