Modelová Otázka ID 2763 - 
Určete objem vzduchu, který obsahuje právě 3 kg kyslíku (objemový zlomek kyslíku ve vzduchu je 0,21).
Správná odpověď je: D
Nejprve musíme vypočítat, jaký objem mají 3 kg kyslíku. Z hmotnosti a molární hmotnosti O2 (32 g/mol) zjistíme látkové množství, z látkového množství poté pomocí molárního objemu ideálního plynu vypočítáme objem kyslíku. Objemový zlomek je podobný hmotnostnímu zlomku, pouze se v něm počítá s objemy. Jedná se tedy o podíl objemu určité složky a celkového objemu směsi. Objem vzduchu vypočítáme jako podíl objemu kyslíku a objemového zlomku kyslíku ve vzduchu. Výpočet provedeme následovně:
V(O2) = (m/M) · Vid = (3 000/32) · 22,414 = 2 100 dm3 = 2,1 m3
V(vzduch) = V(O2)/0,21 = 2,1/0,21 = 10 m3.
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