The coefficient for aluminium is 4.
Explanation:
We have the following chemical reaction:
Al + O₂ → Al₂O₃
In order to balance the chemical equation the number of the atoms entering the reaction should be equal to the number of atoms leaving the reaction.
2 Al + 3/2 O₂ → Al₂O₃
However we need integer numbers so we multiply everything with 2.
4 Al + 3 O₂ → 2 Al₂O₃
The coefficient for aluminium is 4.
Learn more:
about balancing chemical equations
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