Practical investigations
Read our general notes on Risk Assessment
I am doing a planning exercise to determine the % mass of iron in cast iron by redox titration. Its all ok apart from one section where I have to use calculations to justify the amounts/concentrations used, or just to show what is a suitable amount/concentration.
In my calculations to determine the mass of iron and then the % mass of iron, I have used 0.04 molar KMnO4 and 0.50g of cast iron. I chose these values because 0.50g of iron seemed a sensible amount, and 0.04 molar KMnO4 is a value I found in a question about a similar reaction.
I'm assuming the classic equations such as No. of Moles = Mass/Mr and NOM in solution = volume x concentration must be used, but where do I start?
Ulex says ...You have used your experience to give you a start, which is exactly what you should be doing. Try these steps.
1 Work out the number of moles of iron in 0.5 g (if you specify 0.56 g this works out as 0.01 mol !) assuming for the moment that the cast iron sample is pure iron.
2 The minimum number of moles of sulphuric acid required to dissolve the iron will be the same as this. Aim to use 1 mol dm-3 H2SO4 and provide a generous excess.
3 Put your solution into a 100-cm3 standard flask and make up to the mark with 1 mol dm-3 sulphuric acid (remember why ?). Work out the concentration of the iron(II) ions in the solution.
4 Pipette out 10.0 cm3 portions and titrate with 0.04 mol dm-3 KMnO4 ; Work out from the equation what volume of solution should be required. Is this a reasonable volume, not too much or too little? Now remember that your sample isn’t pure iron so the actual titration value will be smaller than you have worked out. Is this still reasonable ? If your titration volume works out as more than 50 cm3 (say) your KMnO4 is too dilute, if it works out as less than 10 cm3 it is too concentrated – adjust accordingly.
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.
For further guidance see our tutorial on Risk Assessment.
back to Practical investigations
Rate this page or react
Share your views on this page, 1 ratings so far
, rated at 
updated: 14 March 2007
