Yes it need to be like that cause when it like that it like that
2NaClO₃ → 2NaCl + 3O₂
mole ratio of NaClO₃ to O₂ is 2 : 3
∴ if moles of NaClO₃ = 12 mol
then moles of O₂ =
= 18 mol
Mass of O₂ = mol of O₂ × molar mass of O₂
= 18 mol × 16 g/mol
= 288 g
So I wasn't sure which equation to use since you did not specify so I just used the decomposition reaction. If you should have used another reaction then just follow the same steps and you'll get your answer.
Scandium lost 3 electrons and gave them to oxygen the formula would be Sc2O3
The mass numbers of the different isotopes of that element are averaged according to their respective abundances in nature.