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Chemistry Forums for Students => High School Chemistry Forum => Topic started by: iknowashtron on October 07, 2017, 08:38:06 AM

Title: chemistry problem
Post by: iknowashtron on October 07, 2017, 08:38:06 AM
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!
Title: Re: chemistry problem
Post by: chenbeier on October 07, 2017, 08:51:14 AM
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
Title: Re: chemistry problem
Post by: iknowashtron on October 07, 2017, 09:01:26 AM
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.
Title: Re: chemistry problem
Post by: Borek on October 07, 2017, 09:51:35 AM
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.
Title: Re: chemistry problem
Post by: chenbeier on October 07, 2017, 10:05:27 AM
If she works below 20 ° C  maybe. Above it will decompose.

https://en.wikipedia.org/wiki/Iron(III)_sulfide
Title: Re: chemistry problem
Post by: Borek on October 07, 2017, 12:14:37 PM
Quote from: chenbeier on October 07, 2017, 10:05:27 AM
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.