[jopsi]

How can I solve how much K₃[Fe(C2O4)₃]·3H2O will form when reagents are (NH₄)₂Fe(SO₄)₂·6H2O (7,5g), H₂C₂O₄·2H2O and K₂C₂O₄·H2O?

[Arkcon]

First you will need a balanced chemical reaction.  In it, you will have to identify the phase of the reaction -- i mean to say, if aqueous solution, you can start ignoring the crystalline water of hydration.  You will have to be able to relate the units of the balanced chemical reaction, to the grams of reactants you have, and convert to get results for "how much" in whatever units you need.

I realize my explanation is a little obtuse, but you have some starting work to do as well.   That's how things are done here on the Chemical Forums, we try to help you, help yourself.  We've detailed that in our our Forum Rules{click}. You already accepted the rules when you signed up for our forum, and you have to follow them, whether you agree with them or not, or even if you're unaware of them. 

[jopsi]

What is the product in this reaction?

[Fe(C₂O₄)₃]^3- + H₂SO₄ -> ?

[Borek]

You have to show your attempts at answering the question to receive help, this is a forum policy.