Step-by-step explanation:
Law of conservation of mass states that mass can neither be created nor be destroyed but it can only be transformed from one form to another form.
This also means that total mass on the reactant side must be equal to the total mass on the product side.

Atomic mass of aluminum = 27 g/mol
Molar mass of oxygen gas = 32 g/mol
Molar mass of aluminium oxide = 102 g/mol
Total mass on the reactant side :

Total mass on the product side :
![2* 102g/mol=204g/mol]](https://img.qammunity.org/2018/formulas/chemistry/high-school/949gota3al7n0yn2b9s4w544hy84r3ligx.png)
Total mass on the reactant side = Total mass on the product side.