Kolik gramů 35% roztoku kyseliny chlorovodíkové potřebujeme k přípravě 100 g jejího 10% roztoku?
Správná odpověď je: C
Vycházíme‑li ze vzorce pro hmotnostní zlomek, můžeme vyjádřit hmotnost čisté látky jako součin hmotnostního zlomku a hmotnosti celého roztoku. Ředěním nedochází ke změně hmotnosti čisté látky obsažené v roztoku, součiny se pro původní a výsledný roztok musí rovnat. Výpočet tedy bude vypadat následovně:
m(čistá) = m(35%) · W(35%) = m(10%) · W(10%)
m(35%) = [m(10%) · W(10%)]/W(35%) = (100 · 0,1)/0,35 = 28,6 g.
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