Magnesium oxide decomposes into magnesium and oxygen. If 12.09g of magnesium oxide decompese to form 7.29g of magnesium, what mass of oxygen gas is also realeased in the reaction?

Since there is only one species on the left to drive the reaction....,

Balanced equation.

2MgO  --> 2Mg + O2

12.09 g MgO (1 mole MgO/40.31 g)(1 mole O2/2 mole MgO)(32 g/1 mole O2)

= 4.80 grams of oxygen gas produced

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As you can see from the answer the stoichiometry was not really necessary ( except for instructional purposes ) as simple arithmetic could have given the same answer..