Modelová Otázka ID 2749 - 
Jaká je hmotnost uhličitanu sodného, který obsahuje právě 14 g sodíku? Ar(Na) = 23
Správná odpověď je: A
Molární hmotnost Na2CO3 je 106 g/mol. 1 mol Na2CO3 obsahuje 2 moly sodíku. Je tedy nutné vypočítané látkové množství sodíku vydělit dvěma. Výpočet provedeme následovně:
m(uhličitan) = {[m(Na)/M(Na)]/2} · M(uhličitan)
m(uhličitan) = [(14/23)/2] · 106 = 32,3 g.
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