How many moles of #Zn# will be needed to completely react with 0.4 moles of #HCl# in the reaction #Zn + HCl -> ZnCl_2 + H_2#?
The equation above is stoichiometrically balanced. What does this mean? It means that for every reactant particle, there is a corresponding product particle. Stoichiometry is also practised in commerce: for every debit item (each charge made to an account), there must be a corresponding credit item (a deposit to made to another account). This principle of "rob Peter, and pay Paul" is fundamental to stoichiometry.
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