How to calculate moles?

Before we start, I know the formula: Moles = Mass/Mr

A compound has the formula M(OH)2, and it contains 32.8% oxygen by mass. M is an unknown metal which I am supposed to find. (That's not I'm asking though, I know M is copper.)

My question is what numbers do I use to calculate a mole?
e.g.if I were to find how many moles of Oxygen there were in this compound, would I...

-> do 32.8/16 (Ar of Oxygen =16) = 2.05
or
-> do 32.8/32 (Mr of O2 = 32) = 1.05?

Then, when comparing moles of (OH) to moles of M, are there two times the moles compared to M or are the number of moles of (OH) the same as number of moles of M?

mole of oxygen ATOMS is /16

so does that mean that in all of these questions i should disregard the subscripts? (eg diatomic molecule)

You can think of it like:

M+2O+2H-->M(OH)2

Which is why you'd only consider the Ar of Oxygen for these types of questions, although that is not the correct reaction, it gives you a guideline of what moles ratio you are working with.

So then to clarify, I could say that whenever elements are in brackets, I use their Ar?
And that in this case there are two times as many moles of hydroxide than the metal M?

They don't necessarily have to be in brackets, Ar is used to find the empirical or molecular formula, and in this case a given compound's Mr.

Say if you had 50g of O2, and you wanted to find the moles, you would have to use Mr. And yes there are two times as many moles of OH than M

Sorry for more questions, but just to clarify, in M(OH)2, there are 32.8/16 = 2.05 moles of oxygen and not 32.8/32= 1.025? But why? Because the way I see it, there are two oxygen atoms in the compound (or are there?).

(I hope that doesn't make me sound stubborn, I'm open to learning and I want want to know the main principle behind this so I can master the whole topic)

Thanks for all your help so far!

Yes there are 2.05 moles of oxygen (O) in the compound, the second calculation - 32.8/32 - calculates the moles of O2 in the compound.

The question tells you that there are 32.8% of oxygen (O) in the compound, not O2, and so to work out the moles oxygen, you divide by 16 and not 32.

Strickly speaking the number of oxygen atoms in the compound would be 6.022x10^23 multiplied by the number of moles of O in the compound, which is why we work with moles and not number of atoms. You are correct in thinking that for every 1 molecule of M(OH)2 there are 2 oxygen atoms, but since we're working with masses, we must take the number of moles into consideration. The formula M(OH)2 tells you that there are 2 moles of oxygen, 2 moles of hydrogen in the compound and 1 mole of M

No worries for the questions, if you have more please feel free to ask, hope this explanation wasn't too confusing.

Yes it would be common sense to know that they are talking about O and not O2.

The big number in front, tells you the relative number of moles for a compound compared to another. For example: N2+3H2-->2NH3. The big numbers in front tell you that for every mol of N2, there are 3 mol of H2, so yes you would still need to take that into consideration. The difference here is that you are dealing with a reaction rather than just a compound, but the same sort of principle takes place, I certainly would not ignore it, but depending on the question, you would know which bits of information are needed.

Right! I get this now. Thanks so much for all the help and I'm sorry about the silly questions :3 I would rep you now, but I can't until I rep others first >.>