Modelová Otázka ID 2744 - 
Kolik gramů vápníku je obsaženo v 15 g oxidu vápenatého? Ar(Ca) = 40
Správná odpověď je: D
1 mol CaO obsahuje 1 mol Ca. Molární hmotnost CaO je součet molárních hmotností Ca a O, tedy 56. Vypočítáme látkové množství CaO (podíl skutečné hmotnosti látky a její molární hmotnosti), toto látkové množství je stejné jako látkové množství vápníku. Následně lze pomocí stejného vzorce vypočítat hmotnost vápníku jako součin vypočítaného látkového množství a molární hmotnosti vápníku. Výpočet provedeme následovně:
m(Ca) = [m(CaO)/M(CaO)] · M(Ca)
m(Ca) = (15/56) · 40 = 10,7 g.
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