Mass of iron chloride (FeCl₂) : 22.68 g
Proust stated the Comparative Law that compounds are formed from elements with the same Mass Comparison so that the compound has a fixed composition of elements
In the same compound, although from different sources and formed by different processes, it will still have the same composition/comparison
With this law, we can calculate how many grams an element is needed to make a compound with a certain mass, as desired
[tex]\tt mass~Fe=\dfrac{Ar~Fe}{MW~FeCl_2}\times mass~FeCl_2[/tex]
The iron ions in 50 g mixture :
[tex]\tt 20\%\times 50~g=10~g[/tex]
[tex]\tt 10=\dfrac{56}{127}\times mass~FeCl_2\\\\mass~FeCl_2=22.68~g[/tex]