Practical investigations
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How can I determine the percentage purity of magnesuim carbonate in a digestion tablet containing this compound using both thermal decompostion and the reaction with an acid?
Ulex replies
You will have to assume that the tablets contain only one compound, the magnesium carbonate, which decomposes on heating and which reacts with hydrochloric acid. It is quite important to make a point about this in your write-up.
Thermal decomposition
Weigh a sample of the tablet (crush it first). Heat it strongly to decompose it into magnesium oxide and carbon dioxide. Let it cool and weigh the solid residue. How do you know whether all the magnesium carbonate has decomposed? Answer, heat it again, cool and re-weigh. If it doesn’t get any lighter in mass, it was fully decomposed. If it does you will have to repeat the heating, cooling and re-weighing until it reaches a constant mass. The loss of mass compared with the original is the mass of carbon dioxide produced. Use the equation to work out from this the mass of magnesium carbonate which produces this amount of carbon dioxide. Compare this with your first weighing and find the percentage purity.
Reaction with acid
You could weigh another sample and put it in an apparatus consisting of a stoppered side-arm test tube connected to a gas syringe. Add hydrochloric acid (work out roughly how much to use), quickly replace the stopper and collect the carbon dioxide produced. Measure its volume. Use the equation to work out the mass of magnesium carbonate produces that volume of carbon dioxide. Compare this mass with your weighing and find the percentage purity, as before.
Alternatively you could use a titration method to see how much of standard hydrochloric acid the sample will neutralise.
This suggests some basic methods but I have not given you any detail – this you must figure out for yourself, bearing in mind what you have to do to achieve accuracy.
If you adopt these methods you must also acknowledge the help you have been given by www.chemistry-react.org when you write up your results – it would be dishonest to do otherwise.
Risk assessment
Before attempting any practical work based on the advice and suggestions on this website, you must do the following. Identify any hazards, assess the risks from these hazards, and then decide appropriate control measures to reduce the risks. You must have these approved by those in authority in your school or college laboratory. Do not rely on what is said on this website.
For further guidance see our tutorial on Risk Assessment.
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updated: 25 January 2006
