Number of atoms reacting with a gas

[dacoolguy]

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?

 

[Borek]

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.