[Kushina]

In 4 different volumetric flasks at 4 different temperatures, 10mL of acetone and 10mL of 0.5M HCl was added, and diluted to 100mL mark. 10mL of 0.05 M Iodine solution was pipetted into the mixture and left to stand. Total volume of solution = 110 mL. Hence initial concentration of iodine = ([(0.05/1000) x 10]/110) x 1000 = 4.55E-3 M. After 5 min, 10mL of reaction mixture was pipetted against 0.005M sodium thiosulfate. This was repeated every 5 min for 20min, therefore i have 4 titer values for each temperature. From that i calculated the number of moles of remaining iodine in 10mL of reaction mixture.
n(moles of iodine) = 0.5 x no of moles of thiosulfate
that's in 10 mL of reaction mixture,
so to calculate the concentration of iodine remaining, it was to divide it by 10 and multiply by 1000. For room temperature and 30 degrees, the calculation worked well and i plotted a graph of conc of iodine remaining against time to find the rate. BUT for 15 degrees and for ice bath temp, the concentration of iodine remaining (from my calculations) become higher than initial concentration of iodine and that's impossible. What am I doing wrong?

[Borek]

Hard to say not seeing your numbers.

However: have you tried the titration of iodine without acetone? Perhaps the concentrations of iodine and thiosulfate are not what you think they are?

[Kushina]

Apparently, the concentration is supposed to be as stated because it is the lecturer who prepared it. Since at room temperature and 29 deg, the rate of reaction of iodine with acetone is higher, less iodine is left so volume of thiosulfate that was required was less than 17mL.
For 15deg, the volumes at diff times were: 19 mL, 16.2mL, 13.1mL and 10 mL. When i calculated for the 19mL, i got conc of iodine 4.75E-3 M but my initial conc is 4.55E-3.
For 0 degrees, the volumes of thiosulfate are: 31.50, 27.80, 23.00, 22.60. For all those volumes, the concentration of iodine remaining become higher than initial.

[Borek]

I see nothing wrong with your calculations, following your procedure and concentrations given titration of iodine without any reaction should require 18.2 mL of the thiosulfate solution.

Obviously something is wrong, but the information you presented and the calculations seem to be consistent.

[Kushina]

Okay, thank you for your help...Maybe the lab attendents gave us the wrong concentration of solutions..