We were measuring the amount of O2 dissolved in water by Winkler's method. Here's a link to the procedure and reactions: (The same is attached to this post) We had to use this formula: c(O2) (mg/L)=(V1×N×E×1000)/V4 where V1 = Vol of titrant Na2S2O3 V4 =Vol of analyte E =Equ wgt of O2 N =Normality of Na2S2O3 sol Now I've got two questions: How do I calculate Equivalent weight of O2? Is it from the 2nd reaction where n=2 as O2 is being reduced to O2-? How's this formula V1S1=V2S2 going to calculate the conc. of O2 straight way when Na2S2O3 reacts with I2 and not O2? Through this equation aren't we supposed to relate only the Volume and strength of chemicals that react ? O2 and N2S2O3 are not directly reacting!