Error Analysis (1 Viewer)

[Cab31]

I did an experiment where i needed to calculate the equilibrium constant (Kc) in two different mixtures of the same thing (but different volumes of the substance)

Mixture one:
CHCl3 5ml
I2 saturated CHCl3 solution 15ml
0.075 M Kl solution 200ml

Mixture two:
CHCl3 10ml
I2 saturated CHCl3 solution 10ml
0.075 M Kl solution 200ml


What we did was shake them up, then wait until there was an aqueous layer and a chloroform layer. Then in each sample, we titrated 5mL of the chloroform and 10mL of the aqueous layer.
they were titrated with sodium thiosulfate - which was 0.00500M + or - 0.00005M

the equilibrium equation was:

I2 + I- (two arrows either way) I3-

to find Kc it was:

Kc = [I3-] aq
[I2]aq [I-]aq

I understand (on a very basic level) what is going on here. My problem is that i have NO idea how to go abut this error calculation stuff. It is so confusing! There is all these delta signs and i dont know whether to use relative or absolute error. If any one knows some chemistry and could help, that would be really great!!!

 

[Jumbo Cactuar]

I am guessing this is almost, but not quite, entirely unrelated to HSC chemistry, but I'll help where pos.

Initally;
solvent>solutes
CHCl3> I2
H2O> I-

Shakey shake shake

CHCl3> I2
H2O> I- , I3 -




I am guessing you used a starch indicator for the titration which weakly bonds to I2 & I3- to form coloured complexes. This is then titrated w/ thiosulfate standard and all the I2 & I3- is reduced to I-, even the complexed I2 & I3-. End point is colourless because there is no more coloured complexed starch left.

titration of chloroform layer:
2 S2O3 2- + I2 --> 2I- + S4O6 2-
hence amount I2 in chloroform = 2 * amount S2O3 used.

titration of aqueous layer:
2 S2O3 2- + I3 - --> 3I- + S4O6 2-
hence amount I3 - in water = 2 * amount S2O3 used.



You started with:
mix 1:-
amount(I3 -) = 0 moles
amount(2 -) = unknown
amount(I -) = 0.015 moles
mix 2:-
amount(I3 -) = 0 moles
amount(2 -) = unknown
amount(I -) = 0.015 moles

You ended up with:
mix 1:-
amount(I3 -) = n moles
*as calculated above*
amount(2 -) = known
amount(I -) = 0.015 - n moles
mix 2:-
amount(I3 -) = m moles
amount(2 -) = known
amount(I -) = 0.015 - m moles

So you can work out two values of Kc and pray that they are the same, or roughly the same.


Now for errors relating to the standard. This is how I would do it. As I assume you are in first year, they want only the contribution of error based upon the distribution limits of thiosulfate. Now that range could be where it definitely going to lie, or the 95% confidence of where it is going to lie. I'd say it's a 95% confidence interval because it less precise and we're more likely to get it right;

+- 2 sigma = +- 0.00005 M
sigma = 0.000025 M = u
RSD = u / mean
= 0.000025/0.005 *100 %
= 0.5 %

Now all the three measurements in our Kc calculation are related to thiosulfate concentrations;

RSD(Kc) ^2 = RSD(I-)^2 + RSD(I2) ^2 + RSD(I3-) ^2
= 3*(0.5)^2
= 0.75
RSD(Kc) = 0.87%

so Kc = Kc mean +- 2*0.87%
Kc = Kc mean +- 1.73%
(I'd evaluate the 1.73% of the mean)

Uh, not studying your course I have no idea what they are looking for. But like I say, this is how I'd do it.

Hope it wasn't urgent