[iknowashtron]

If i have 100mL of the solution iron(III) chromate having a concentration of 0,620 mol/L.
All Fe3+ ions are precipitated by adding to this solution a certain volume of aluminum sulfide solution of 1,00mol/L.
Considering that the reaction is complete, that there is no excess and that the volumes can be added, how do I calculate the concentration of Al3+ ions after precipitation.
I think I completed and balanced the equation correctly, but I’m not sure:
2Fe2(CrO4)3 + Al2S3 = Fe4S3 + 2Al(CrO4)3
Then I think I'm supposed to use c = n/v somehow to calculate the concentration?
Could someone walk me through what i'm supposed to do? Thank you!

[chenbeier]

Who had given this exercise. Because almost all substances not existing as discribed. Aluminiumsulfide is not solouble in water and if yes there is decomposition to hydrogen sulfide.

But if we think all is existing.

Calculate first the moles  for iron. Then the moles for sulfide.  Check which iron sulfide is existing. You have an redoxreaction.

The consumption of sulfide reflects of the aluminium ions

[iknowashtron]

thank you for taking the time to answer!
And i'm assuming my teacher came up with this question, i'll have to talk about it with her.

[Borek]

Definitely a poor selection of compounds, but my take is that you are expected to follow the stoichiometry. As it is all extremely simplified I wonder if you should not have Fe₂S₃ as the precipitation product.

[chenbeier]

If she works below 20 ° C  maybe. Above it will decompose.

https://en.wikipedia.org/wiki/Iron(III)_sulfide

[Borek]

If she works below 20 ° C  maybe. Above it will decompose.

https://en.wikipedia.org/wiki/Iron(III)_sulfide

Technically you are right, but you are missing the point - whole exercise is designed not around a real chemistry, but around how-I-imagine-simplified-chemistry. That typically means extrapolating compound formulas and compound properties from the most basic ones. Soluble Na₂S and CaS get extrapolated to a next sulfide Al₂S₃. Transition metals sulfides are known to be insoluble, so Fe(III) will precipitate - but its formula must follow A₂B₃ pattern, and so on.

Yes, this logic is flawed, sadly, it is not that rare between teachers that have only very rudimentary chemistry knowledge.