The limiting reactant in a chemical equation is the reactant that is completely consumed when the chemical reaction takes place. As a result the reactant determines for how long the reaction takes place.
The reaction given is that between aluminum and oxygen to yield Al2O3. The equation of this chemical reaction is:
4Al + 3O2 --> 2Al2O3
3.17 g of Al and 2.55 g of O2 are available for the reaction to take place. The molar mass of Al is 27 and that of O2 is 32.
3.17 g of Al is equivalent to 0.11 moles and 2.55 g of O2 is equivalent to 0.079 moles
As 4 moles of Al react with 3 moles of oxygen in the reaction, there should be 3/4 times as much oxygen as aluminum. But the number of moles of the reactants available to react shows that a lesser amount of oxygen is available.
In this case, oxygen is limiting reagent and the reaction stops when all the oxygen is consumed.
First step balancing the given reaction.
4Al + 3O2 -----> 2Al2O3
Second step Find moles from the given mass.
3.17 gm of Al
Moles = mass / molar mass
Moles = 3.17 / 27
Moles = 0.11
2.55 gm of O2
Moles = mass / molar mass
Moles = 2.55 / 32
Moles = 0.07.
Moles of Al = 0.11,
Moles of O2 = 0.07
Third step Divide the Moles with the stoichiometric coefficient in the given chemical reaction.
For Al = 0.11/4 = 0.0275
For O2 = 0.0233
From the above two value we can see that O2 has less value that is 0.0233, so O2 is the limiting reactant for this reaction.