Number of atoms reacting with a gas

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SUMMARY

The discussion centers on a chemistry problem involving the reaction of aluminum and oxygen to form aluminum oxide, represented by the equation 4Al + 3O2 = 2Al2O3. The user incorrectly calculated the number of oxygen atoms reacting with 45 dm³ of O2 gas, initially arriving at 1.24 x 10²⁴ atoms. The error stemmed from not accounting for the molecular composition of O2, which consists of two oxygen atoms per molecule, leading to the conclusion that the correct number of reacting oxygen atoms is actually 4. This highlights the importance of understanding molecular formulas in stoichiometric calculations.

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Homework Statement


4Al + 3O2 = 2Al2O3
What number of O-atoms will react if 45dm3 of the gas reacts with an excess of Al?
The answer is apparently: 4 atoms

Homework Equations





The Attempt at a Solution


Using the formula n=V/Vn (or the law stating that any gas occupies 22.4dm3 at STP), I found that the number of moles was (225/112) or 2.01. Then I used the formula: n=n*Na (number of moles times Avagadro's Constant) giving me 1.24*1024 atoms. Where did I go wrong please?
 
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The answer is not 4 atoms.

I have no idea what 225/112 means, even if the number of moles seems to correct.

You have 45 L of the gas, there is 22.4 L per mole - how many moles of the gas?

You have correctly converted number of moles of gas to number of molecules, however, you forgot that there is more than one atom per molecule.
 

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