Chemical quantities (AS)
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Pages 13-14 in the Nuffield Student Book describe the preparation of a hydrated salt, ammonium iron(II) sulphate from iron filings, ammonia solution and sulphuric acid. Why is the actual yield of crystals likely to be much lower than the theoretical amount?
When the solution cools and crystals form, quite a lot of the salt remains in the solution and doesn’t crystallise. It may be, however, that your solution dried out completely before you were able to inspect it. If this is so, you have no guarantee that you allowed enough water for the water of crystallisation so, again, you would not get as much as you thought you ought to get.
Also note that there are inevitable losses when you carry out any practical preparation. It is impossible to retain all the product when filtering, pouring from one container to another and when separating crystals from a solution.
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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: 27 August 2003
