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Reacting masses mole question

image.jpgI need help on how to work these type of mole questions. I have a rough idea on how to work it out. For example when working out the moles of the known substance ( 2 Al) it says that its burned under oxygen so when working out the number of moles in order to later find out the number of moles of the unknown would I include the relative molecular mass of oxygen and add it to the relative molecular mass of Aluminium too or would I just use the mass of Aluminium as I did in my answer?
1) Convert Mass to moles
2) Use the chemical equation to relate amounts in moles
3) Convert moles to mass

1) First find the moles of aluminium (they give you the mass so this is how you know you find the moles of aluminium)
Moles = mass/RFM
Moles = 135/27
Moles = 5

2) You need to find moles of Al2O3, So use ratios!.
There's 2Al to make one Al2O3.
Ratio = 2:1, therefore 5moles : 2.5moles
You now know that Al2O3 has a mole value of 2.5

3) Using the same equation, rearranged
Mass = Moles X Rfm
Mass = 2.5 X 102 (102 is the molar mass for aluminium oxide)
Mass = 255g

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