Modelová Otázka ID 2816 - 
Kolik gramů chloridu sodného potřebujeme k přípravě 20 g 30% roztoku?
Správná odpověď je: D
Pro výpočet můžeme použít vzorec pro hmotnostní zlomek, ze kterého vyjádříme hmotnost složky. Hmotnostní zlomek dosazujeme jako číslo, nikoli v procentech. Výpočet tak bude vypadat následovně:
W = m(NaCl)/m(roztok)
m(NaCl) = W · m(roztok) = 0,3 · 20 = 6 g.
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