Often more than one reaction is required to change starting materials into the desired…

Often more than one reaction is required to change starting materials into the desired product. This is true for many reaction that we carry out in the laboratory and for many industrial process. These are called sequential reactions. The amount of desired product from each reaction is taken as the starting material for the next reaction.
I : 2KClO₃2KCl + 3O₂
II : 4Al + 3O₂ 2Al₂ O₃
KClO₃ decomposes in step I to give O₂, which in turn, is used by Al to form Al₂O₃ in step II. First we determine O₂ formed in step I and then Al used by this O₂ in step II. Both reactions can be added to determine amount of KClO₃ that can give required amount of O₂ needed for Al.
Net : 2KClO₃ + 4Al 2KCl + 2Al₂O₃
Thus, 2KClO₃ ≡ 4Al
Or KClO₃ ≡ 2Al.
I : CaO + 3C CaC₂ + CO
II : CaC₂ + 2H₂O Ca(OH)₂ + C₂H₂
CaC₂ (calcium carbide) is prepared in step I. It is used to prepare acetylene (C₂H₂) in step II. Suppose we want to determine amount of CaO that can give enough CaC₂ to converted required amount of C₂H₂. Amount of CaO is determined in step I and then amount of C₂H₂ in step II. We can relate CaO and C₂H₂ stoichiometrically by writing net reaction which is
CaO + 3C + 2H₂ O Ca(OH)₂ + C₂H₂ + CO
Thus, CaO ≡ C₂H₂
120 gm Mg was burnt in air to give a mixture of MgO and Mg₃N₂. The mixture is now dissolved in HCl to form MgCl₂ and NH₄Cl, if 107 grams NH₄Cl is produced. The reaction are follows
I. Mg + \(\frac{1}{2} \mathrm{O}_{2} \longrightarrow \mathrm{MgO}\)
II. \(3 \mathrm{Mg}+\mathrm{N}_{2} \longrightarrow \mathrm{Mg}_{3} \mathrm{~N}_{2}\) 
III. MgO + 2HCl MgCl₂ + H₂O,
IV. Mg₃ N₂ + 8HCl 2NH₄Cl + 3MgCl₂
Then the moles of MgCl₂ formed is : (At. wt. Mg = 24, N = 14, Cl = 35.5)