# Job's Plot or Method Of Continuous Variations

I was wondering why in order to find the stoichiometry of a complex using Job's Plot, the two compounds must have the exact same molarity. What kind of effect would it have if molarities weren't the same? Thanks in advance!

• If A and B were reactants, then a peak at a mole fraction of 0.5 A would indicate 1:1 complex, a peak at 0.25 A would indicate a $\text{AB}_3$ complex.
– MaxW
Feb 3, 2017 at 19:27
• When using for example NiSO4 and Na2EDTA the peak is observed at 0.5A so the complex is 1:1. But why do we have to use the same molarity of the two compounds? Does it have to do with absorption or obedience to Beer's Law? Feb 3, 2017 at 19:37
• Huh?!? in my example the mole fraction of A would be plotted from 0% A (100%B) to 100%A (0%B). So the plot doesn't "use the same molarity." The peak occurs at the mole fraction which gives the stoichiometry of the complex.
– MaxW
Feb 3, 2017 at 22:10
• I agree with you as to where the peak occurs and that it gives us the stoichiometry. In order to be specific I'm going to use the same example of NiSO4 and EDTA 2- .After we apply Job's Plot the peak gives us the stoichiometry which is 1:1. The question is why the solutions of NiSO4 and Na2EDTA have the same concentration before we mix them. Feb 3, 2017 at 22:34
• ?!? Maybe because it makes the math easy... Assume Ni is 0.1 molar and EDTA is 0.1 molar. Taking $x$ mls of Ni and diluting to 100 mls with EDTA yields a solution which has a mole fraction of $x$% Ni.
– MaxW
Feb 4, 2017 at 0:05