How many moles of X ions/atoms in a compound?

I'm not sure how to work this out...

I don't want a simple answer either. I just want to know what kind of steps I should do to work out these questions. Should I work out the Mr of the atoms/ions and then divide by the total for all of them?

11
a) In 20 moles of Al2O3
i) how many moles of Al3+ ions?
ii) how many moles of O2- ions?

b) In 360g of water
i) how many moles of H atoms?
ii) how many moles of O atoms?

This isn't urgent because it is the half term, but I'm hoping for some guidance

Part a is quite simple, no equations required. If you had 10 moles of H2O molecules, then for every 1 molecule you would have 1 Oxygen atom, so you can see that you would have 10 moles of Oxygen atoms. Hopefully you can also see that each molecule has 2 Hydrogen atoms so 10 moles of water molecules has 20 moles of Hydrogen atoms.This is an example of the fact that the moles of each atom are proportional to the number of atoms in the formula unit. Al2O3 is an ionic solid (rather than a covalent compound like water) so it consists of Al3+ and O2- ions but the same logic can be applied.

For part b, if the question gives you grams and is asking about moles, the first thing to do is to work out the number of moles.

To do this, first you need Mr (relative molecular mass) then you need to use the formulan = m/ Mr

n is the number of moles
m is the mass
Mr is the relative molecular mass

once you've worked out the number of moles of water in 360grams you can do question b the same way as part a.

Thanks Just to clarify would the answer to the first one (using the explanation you given) be:


11
a) In 20 moles of Al2O3
i) how many moles of Al3+ ions? 40 moles
ii) how many moles of O2- ions? 60 moles