Practical investigations
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I am currently writing my coursework, and I have to find the percentage purity of MgCO3 using a back titration, I have found out a basic method for it, using hydrochloric acid and sodium hydroxide, but I don't understand the calculations to find out how much of each substance to use and at what concentration. Also I don't understand any of the other calculations?
Key steps:
(1) Weigh out accurately a sample of magnesium carbonate.
(2) Dissolve the carbonate in a measured excess of acid (using a measured volume of a standard solution)
(3) Titrate to find the amount (in moles) of acid in excess.
From (2) and (3) you know the amount (in moles) of acid that reacted with the measured mass of carbonate (the sample).
From the equation you know that 1 mol magnesium carbonate reacts with 2 mol HCl(aq). So you can work out the amount of the carbonate in your sample.
Since you know the molar mass of magnesium carbonate you can work out the mass of carbonate in the sample and hence the purity.
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: 04 April 2005
