Practical investigations
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I am trying to find the percentage purity of cast iron using a redox titration with potassium manganate(VII). I plan to first dissolve the iron in 1 molar sulphuric acid but how do you determine what amount of cast iron to crush and dissolve if you do not know its formula mass?
Igloo writes
This is a good question. The best thing to do is to assume that cast iron contains 100% iron, and to carry out your calculations on this basis. As a result you are bound to have sufficient acid to oxidise all the iron.
The equation is: Fe + H2SO4 -> FeSO4 + H2
Now, if you crush, shall we say, exactly 10.0 g of cast iron, on the assumption that all of this is iron, you will be using 10.0/55.9 mol of iron = 0.179 mol
Thus from the equation 0.179 mol of sulphuric acid will be required.
Since the acid is 1.00 molar, 0.179 mol is present in (0.179 x 1000) = 179 cm3 of solution.
Obviously you need an excess of acid, so you can use 250 cm3, or perhaps 500 cm3.
Remember, all you need is an excess, and since the cast iron is obviously not 100% pure iron, you will have more of an excess than necessary, but this won’t matter, since you need an excess in any case for the subsequent titration with manganate(VII) ions.
Always carry out a risk assessment and check with your teacher before starting any practical work.
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.
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updated: 11 May 2006
