[usmelmaz]

S₂O₈^2- + 2I^-  2SO₄^-2 + I₂
I₂ + 2S₂O₃^2-  2I^- + S₄O₆^-2
 
I've performed this iodide clock reaction to determine the reaction rate in the PChem Lab. The procedure was like this:

1. Mix 50 mL of 0.036 M K₂S₂O₈ and 50 mL of KI.
2. After 5 minutes, take 10 mL from this solution and put it into 200 mL cold distilled water.
3. Titrate it with 0.02 M Na₂S₂O₃ until its light yellow. Add starch and continue to titrate until the solution is colorless.

Now, I understand that I use the volume of titrant to find the number of moles of S₂O₃^2- so that I can find these:

n(S₂O₃^2-) =  2n (I₂) and n (I₂) = n(S₂O₈^2-)

My lab manual claims that the volume of the titrated analyte is V=10 mL so that,

[S₂O₈^2-] (reacted) = n (S₂O₈^2-) / 10 mL

[S₂O₈^2-]{initial} - [[S₂O₈^2-]{reacted} = [S₂O₈^2-]{unreacted}

Then it says to proceed by plotting [S₂O₈^2-]{unreacted} vs. time graph etc.

I have no problem with using M₁*V₁ = M₂*V₂ but, I can't figure out the initial concentration of S₂O₈^2-. Here are some of the thoughts that I'm considering:

I started out with 50 mL of 0.036 M S₂O₈^2-. However, when I mixed them with 50 mL of KI, the total volume changed to 100 mL so there should be a dilution. By 50 * 0.036 = 100 *M₂, I believe the new concentration of S₂O₈^2- is 0.018 M. That feels like correct up to now and I understand why I need to do this.

But then after 5 minutes, I took 10 mL of this solution and put it into 200 mL of distilled water to titrate. So by mixing 10 mL of 0.018 M S₂O₈^2- with 200 mL water,
10 * 0.018 = 210 * M₂ , the initial concentration became 0.0008571? I'm lost after this part. Not sure if this is needed for initial concentration because the reaction was already started.

After all, I'm doing my calculations based on the titration data which is performed with 210 mL solution. I feel like either here that total volume should be 210 mL and the manual got it wrong:

[S₂O₈^2- reacted] = n ([S₂O₈^2-) / 10 mL

or

I should consider this 210 mL dilution while calculating initial concentration.

I don't also fully understand the idea behind [[S₂O₈^2-]{initial} - [[S₂O₈^2-]{reacted} = [S₂O₈^2-]{unreacted}
Shouldn't it be  Δ[S₂O₈^2-] = [S₂O₈^2-]{final} - [S₂O₈^2-]{initial} so that I can use the rate equation?

If anybody could help me to clarify where (or why) I'm getting lost, it would be really great. Thanks a lot!

[Borek]

But then after 5 minutes, I took 10 mL of this solution and put it into 200 mL of distilled water to titrate. So by mixing 10 mL of 0.018 M S₂O₈^2- with 200 mL water,
10 * 0.018 = 210 * M₂ , the initial concentration became 0.0008571?

General hint: there are moments when you should not worry about concentrations, more about amounts of the substance present in the sample, as that doesn't change during dilution.