# Stoichiometry to find the concentration of O2?

2 Mn2+ + O2 + 4 OH- -> 2 MnO2 + 2 H2O
***** Use the stoichiometry of the chemical reaction to calculate the concentration (mg/L) of O2(aq) and concentration of O2 (aq) (Molarity).
DI Water (22 degrees C)
Volume Na2S2O8 to reach the equivalence point: 8.7ml

I would appreciate any help, thanks!!!

This is what I have done:
1 mol O2 (2mol MnO2/1 mol O2) * (1 mol I2/1 mol MnO2) * (2 mol S2O3^-2/1 mol I2)= 4 mol S2O3^-2

0.00500 Na2S2O3 (0.0087L)= 4.35*10^-5/4mol= 1.0875 * 10^-5

---------------0.00500 M Na2S2O3 standard solution
---------------II. MnO2 (s) + 2I¯ + 4H+---> Mn2+ + I2 (aq) + 2H2O
III. 2S2O32- + I2 (aq) ---> S4O62-+ 2I

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Borek
Mentor
It is a little bit chaotic, can you explain in more details what the procedure and reactions involved are? Have you used sodium persulfate, or sodium thiosulfate (I guess the latter, unless it was a back titration)?

It is a little bit chaotic, can you explain in more details what the procedure and reactions involved are? Have you used sodium persulfate, or sodium thiosulfate (I guess the latter, unless it was a back titration)?
It's a biological oxygen demand experiment-winkler titration. We used sodium persulfate.

We did two trials:
FOR TRIAL 1:
DI WATER (22 C)
VOLUME Na2S2O8 TO REACH EQUIVALENCE PT = 8.7ml
Find the concentration of O2 (aq) (mg/L)
Find the concentration of O2(aq) (Molarity)

TRIAL 2:
DI WATER (0 C)
VOLUME Na2S2O8 TO REACH EQUIVALENCE PT = 2.55ml
Find the concentration of O2 (aq) (mg/L)
Find the concentration of O2(aq) (Molarity)

Borek
Mentor
Strange, as far as I was able to check Winkler method calls for thiosulfate to titrate iodine. One starts with Mn2+, it gets oxidized by the dissolved oxygen producing brown precipitate (containing Mn3+ and/or Mn4+), this precipitate after adding sulfuric acid oxidizes iodides to iodine, iodine is titrated with thiosulfate. No place for persulfate.

Can you write equations of reactions that take place during the experiment?