I'm currently in year 11 chemistry, and I've recently been doing a back titration method of determining the ethanol levels in wine, and have ran into some issues with standardizing a solution of Sodium Thiosulphate (Na2
) against an Iodide (I-
) solution, so problematic in fact, that not even my chemistry teacher has any idea on what may be occurring and is considering calling up the local university for help etc. However, he said that if I were able to research, hear of, find out (etc etc) an answer to this problem that he would forget about the rest of the titration and would grade me an A no questions asked.
From my calculations, which should be right, I have made a thiosulphate solution 0.1M by adding 6.20g to 250ml of water (the thiosulphate's molar mass is 248.18g/mol)
248.18/10 = 24.818
24.818/4 = 6.2045 ~ 6.20g
So my calculations of molarity should be fine, however when we ran into some errors during the wine titration, my chem teacher decided to check the molarity of the thiosulphate against a standardised solution of iodine which had been derived from the reaction between Potassium Iodate and Iodide and found that my first batch of thiosulphate was 0.2M and my second batch was 0.176M. I was not with him when he did this standardizing as he did this after that lesson, however he did give me the method of which he standardized the solution:
The solution of the KIO3
M with 10ml used, a total of 5*10-5
moles of Iodate used and 2.5*10-4
moles of Iodide, creating 1.5*10-4
moles of Iodine and 1.5*10-4
He then diluted a sample of the sodium thiosulphate solution I hade made up by a factor of 1:5, so it should now have a concentration of 0.02M and used 8.4ml of it with the Iodine until the endpoint was found.
So from here I'm somewhat confused...
I know that I made my solution 0.1M, however according to his calculations my solution is out by 50% on the most leanient percentage errors and taking into account of water of crystalisation, but if theres anything that is wrong with those calculations, does have any suggestions on why my solution is out?
Many Thanks to whoever is able to help me with this problem, as it is an absolute pain in the backside.