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.

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