Should I relate the number of moles between substances when converting weight units?
Good day! I graduated from an undergrad degree of chemistry almost 3 years ago but I'm frustrated that I fail to reconcile these things:
In gravimetric analysis, to get the correct amount for the desired way of expressing the analysis result, we convert the mass of the acquired substance to the desired form, and in doing this, we should relate the number of moles of the acquired substance to the desired substance. For example, if you desire to get the grams $Al$ from a sample which you assayed gravimetrically until you have formed $Al_2O_3$, then you have to use the following formula:
$\text{grams } Al = \text{grams } Al_2O_3 \times \frac{2\times MW Al}{MW Al_2O_3}$
wherein the factor $2$ in the numerator was obtained from the term $\left( \frac{4 \text{ mol } Al}{2 \text{ mol } Al_2O_3} \right)$, which arose from the oxidation reaction $4Al + 3O_2 \rightarrow 2Al_2O_3 $.
Now, the question is, should I also use this method of conversion if I just want to express %Al from a %$Al_2O_3$ concentration of a certain substance, even if I don't plan to do a gravimetric analysis to find it? Also, this method of conversion is reaction-specific, but there could be many different ways, not only single-step chemical equations but also multiple-step ones in which the overall mole relation of the substances in a given chemical equation is different from that of a different chemical equation.
Also, what about the expression of ppm $N$ from ppm $NO_3$ and ppm $NO_2$? Does their conversion method work in the same way in which you have to know the reaction involving $N$, $NO_3$ and $NO_2$ to know their mole relations, knowing that monatomic $N$ is not very easy to generate? In this case, I can only see that the conversion method is based on the number of atoms the primary element in each form not on a certain reaction like thee previous case. 565
But if that's the case, then I have another contradicting case to show:
Another one is that of between $NaOCl$ and $Cl_2$ in disinfection solution formulations. If you base it on the last method I mentioned, then the mole ratio should be 2 mol $NaOCl$:1 mol $Cl_2$ because you only have one $Cl$ atom in $NaOCl$ while $Cl_2$ has 2, but according to all of the reputable resources I have found in the internet, the mole ratio here is 1:1. The reason here is supposed to be because of the chemical equation $Cl_2 + 2NaOH \rightarrow NaOCl + NaCl + H_2O$.
If the answer is supposed to be obvious after all these, or I am just completely missing the point, I hope you would be kind to point it out clearly because it's really bogging me down. 870