Modelová Otázka ID 2745 - 
Kolik gramů síry je obsaženo ve 130 g síranu železitého? Ar(S) = 32; Ar(Fe) = 56
Správná odpověď je: A
Molární hmotnost síry je 32 g/mol, molární hmotnost Fe2(SO4)3 je 400 g/mol. V tomto případě jsou však v 1 molu Fe2(SO4)3 obsaženy 3 moly síry. Je tedy nutné vypočítané látkové množství Fe2(SO4)3 vynásobit třemi. Výpočet provedeme následovně:
m(S) = [m(síran)/M(síran)] · 3 · M(S)
m(S) = (130/400) · 3 · 32 = 31,2 g.
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