[Ai]

I was do in this problem: How much Al2O3 is needed to make 100kg of Al from this reaction:

2Al2O3 -> 4Al + 3O2

I see that there is a 2:4 between the two or 1:2. How can I use this ratio to find the kg of Al2O3 needed (is there a intuitive way to do it)?

[azmanam]

grams known -> moles known -> moles unknown -> grams unknown.

If you're given density or molarity, you can use those as conversion factors to get to and from grams or moles, as needed.

[Ak]

doesnt he just have to convert kg to g, then g to moles, do a mole conversion between Al and Al2O3, then take the number of moles from there and convert to g

[azmanam]

Yup.  That's the essence of the g->mol->mol->g sequence. 

I guess I wasn't totally clear.  I threw in the density/molarity information not because it was needed in this problem, but because it can be used to enter the general g->mol->mol->g sequence.  OP was looking for an intuitive way to approach soichiometry problems, and that's my offering.  If you can get into the g->mol->mol-g sequence, you're on your way.

[Ai]

doesnt he just have to convert kg to g, then g to moles, do a mole conversion between Al and Al2O3, then take the number of moles from there and convert to g

Thank You azmanam and AK. I was trying it out its kg->g then mole/g of Al * mole Al2O3/mole Al * g/mole Al2O3