Basic question about reacting quantities ?

All you're doing is working out the ratio of one element to another in a compound.
For a) work out how many moles of Na there is which = 2.3/23 = 0.1 mol.
For oxygen you have (3.1-2.3)/16 = 0.05 mol.
So for every 2 moles of Na there is 1 mole of oxygen. So empirical formula is $\text{Na} _2 \text{O}$.
Why don't you use this and try the next one for yourself.

Thank you very much but here is where I'm stuck for question b)

b) First we work out how many moles of Fe = 11.2/55 = 0.2 moles
Then for Oxygen = 15.96-11.2 = 4.74g/16 = 0.3 moles

So where do I go from here ... ??

again any help is greatly appreciated .

Thank you very much but here is where I'm stuck for question b)

b) First we work out how many moles of Fe = 11.2/55 = 0.2 moles
Then for Oxygen = 15.96-11.2 = 4.74g/16 = 0.3 moles

So where do I go from here ... ??

again any help is greatly appreciated .