What is the difference between O and O2 in stoichiometry calculations?

[Whalstib]

Homework Statement

Hi,
4Al + 3O2 --->2Al2O3

Given 20g of Al and 15g Oxygen which is limiting factor?

Homework Equations

My question is the difference between O2 and Oxygen. Do I start with a mole of O or O2 in my calcs? Since the question is framed "15g of oxygen" (verbatim from a quiz) should it be inferred it's O2 in which case it's 32g/mol as opposed to 16g/mol, either .47 moles O2 or .94 moles O...?

The Attempt at a Solution

I used the O 16g/mol calc.
15g O2 x 1(mole O)/16 (mm O) x 2(mol Al2O3)/3(mol O) = 0.625 moles Al2O3 produced
20g Al x 1(mole Al)/27 (mm Al) x 2 (mol Al2O3)/4 (mol Al) = 0.37 moles AlsO3 produced

With this approach Al is limiting factor. Professor briefly touched upon this problem and I had not chance to question. he was clear that O was limiting factor.

is it my O vs. O2 calculations?

Should my O calc be:

15g O2 x 1(mol O)/32 (mm O2) x 2(mol Al2O3)/3(mol O) = 0.312 moles Al2O3 produced.

Thanks!

Warren

 

[Borek]

While 15 g of oxygen is 0.47 moles of O₂ and 0.94 moles of O, it is still the same number of atoms (that is, amount of oxygen has not changed). However, your reaction is balanced with O₂ - so you should use moles of O₂ for calculation. If you use reaction balanced with O (that is, 2Al + 3O -> Al₂O₃) you use moles of O. That's not reasonable approach as oxygen is diatomic, but it will still give correct result.

And yes, Al is a limiting reagent.

--

 

[Blueshift5]

Hello Warren,

Thank you for your question. It is important to understand the difference between O and O2 in stoichiometry calculations. In this case, the equation shows that 3 moles of O2 are required to react with 4 moles of Al to produce 2 moles of Al2O3. Therefore, it is correct to use the molecular weight of O2 (32 g/mol) in your calculations. Your calculation for the number of moles of Al2O3 produced using O2 (0.312 moles) is correct.

The reason why your professor may have said that O was the limiting factor is because in this particular reaction, both Al and O2 are present in excess. This means that even though there is not enough O2 to react with all of the Al, there is still enough Al to react with all of the O2. Therefore, in this case, O2 is not the limiting factor and your calculation is correct.

I hope this helps clarify the difference between O and O2 in stoichiometry calculations. Keep up the good work in your studies!