Magnesium reacts with oxygen gas forming magnesium oxide as shown;
2mg(s)+o2(> 2mgo(s)
what mass of o2(g) is needed to completely react with 5.00 g of mg?

Molar mass:

O2 = 16 x 2 = 32.0 g/mol              Mg = 24 g/mol

2 Mg(s) + O2(g) --->2 MgO(s) 

2 x 24.0 g Mg > 32 g O2
5.00 g Mg > ( mass of O2)

mass of O2 = 5.00 x 32 / 2 x 24.0

mass of O2 = 160 / 48

= 3.33 g of O2

hope this helps!

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