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# Back Titration watch

1. I'm using a back titration in my coursework for "finding the most effective method for accurately determining the amount of aspirin present in a solution" along with two other methods, and I've just realised I don't actually know how to do a back titration or how to use the results...

Basically, it is "an analytical method which allows us to find the concentration of a reactant of an unknown concentration by reacting it with an excess volume of another reactant of known concentration", but can anyone tell me what I have to do in terms of mole calculations to find the concentration of aspirin??

2. (Original post by Emma205)
I'm using a back titration in my coursework for "finding the most effective method for accurately determining the amount of aspirin present in a solution" along with two other methods, and I've just realised I don't actually know how to do a back titration or how to use the results...

Basically, it is "an analytical method which allows us to find the concentration of a reactant of an unknown concentration by reacting it with an excess volume of another reactant of known concentration", but can anyone tell me what I have to do in terms of mole calculations to find the concentration of aspirin??

Do a normal titration, your aim is to find out something about an unknown sample. So, from the final titration results, work out mole of reactants(your sample probably), but remember you must use a balanced equation.
3. Im on OCR A2 salters course (Chem B). so we have to endure a lovely individual investigation- sinse mine was on vitamin C concentration of Peppers, ive learnt a lot about back titrations. You have to add excess amount of one substance to another in order to process the forward reaction. Its excess because you have to make sure that all of the first reactant reacts. Basically, if you have low amount of moles of one substance A - and you add it to a higher concentration/ volume of substance B, the useful product formed, substance C - its molarity would be directly reliant on both substances A and B, so moles of substance C would be directly proportional to the moles of A used. "B is excess."

In my case, I added Excess Iodide Ions(IO3-) to Iodate ions(I-) in order to make my iodine solution of I2 (in realty its I3- or triiodide, but for the sake of making the equation simple):

(IO3-) + 5(I-) + 6(H+) ==> 3(I2) +3(H2O)

in this case, my excess (substance B) is the I- ions. and i add 400ml of IO3- at 0.01moldm^-3 and just 100ml of 1moldm^-3 of I- to make I2 solution. the moles of I- wont matter, nor will the H+ ions. (the H+'s is provided by sulphuric acid if you're wondering). The only thing affect the moles of I2 is IO3-

400ml of 0.01 mol dm^-3 makes 400/1000* 0.01 = 0.004mols
this reacts with excess (more moles than needed) of I- to make 3 times the moles of Iodate ions. so the moles of the Iodine in solution would be 3* 0.004 = 0.012 moles.

The titrating solution would be in my case, sodium thiosulphate which would remove the brownish iodine color (in presence of starch) to give a clear , colourless solution.

2(S2O3) 2- + I2 ==> 2I- + (S4O6)2-

Sodium thiosulphate used to make the brown colour dissapear would be recorded.. using values to 0.1 etc.. yada yada.. means etc. then this value would then be used to find out how much moles were used of the thiosulphate.. this would then tell us how many moles of iodine it converted back to iodide ions. We'd know how many moles were present in the solution in the first place- so the the moles of iodine removed by the thiosulphate would be taken away from the total amount in the known sample of iodine made. the result- a certain amount in moles of the iodine must therefore have been reduced by the vitamin C of the pepper. therefore, it follows that the amount in moles of the iodine left must be directly proportional to the amount of vitamin C:

C6H8O6 + I2 ==> C6H6O6 + 2I-

so we can work out the moles of the vitamin C using the leftover I2 molar amount.

we can then work this out in terms of grams per decimentre cubed or moles per decimetre cubed, or percentage mass of pepper, in milligrams per 100grams pepper etc. etc.

hope that helps somewhat.
4. Thanks for the post funkmaster but I'm afraid the thread is over a year old

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