Practical investigations
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How is it possible to determine the percentage purity of MgCO3 in an indigestion tablet by a) thermal decomposition and b) reaction with acid? I understand that I can collect the CO2 that is given off in both reactions, but CaCO3 is also present in an indigestion tablet and this will also give off CO CO2, so how can I get just the CO2 from MgCO3?
Igloo writes
If an indigestion tablet consisted simply of a mixture of magnesium and calcium carbonates this would be a fairly straightforward exercise. You would determine the volume of CO2 produced by a tablet (or tablets) of known mass, and then carry out two extra experiments, one using the same mass of pure magnesium carbonate and the other using the same mass of pure calcium carbonate. The results of these latter two experiments could then be interpolated to find the relative proportions of each of the components in your mixture. Alternatively, a calculation method involving algebra could be used with the results from your first experiment.
However, in practice, the tablet is likely to contain “fillers” and other material, which introduce another major variable into the equation. In short, if this is the case, an experiment to measure the volume of CO2 given off, either by heating or by treatment with acid is simply not going to give you enough information to solve the problem. There is no way of “preventing” one of these carbonates from yielding its CO2, whilst the other is allowed to do so.
The only way of resolving this issue is to use a technique where calcium and magnesium differ in their properties, and the only relatively straightforward one which I can think of – which could be carried out in a conventional school laboratory - is to precipitate out the calcium as its highly insoluble oxalate, CaC2O4, and then perhaps to weigh this washed and dried precipitate. This would be a method known as gravimetric analysis. THis would be hard to do accurately.
Incidentally, if you intend to follow this route you need to be aware that oxalates (ethanedioates) are highly poisonous, so it is vital that risk assessments are carried out beforehand and that permission is sought from your teacher before starting any experimental work.
Risk assessment
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updated: 25 January 2006
