Confused about subscripts and coefficients wrt forumulas

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instantresults
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What exactly is the difference between something like (XY)2 and 2XY? I get that the 2XY would indicate 2 moles of XY, but I don't see why (XY)2 wouldn't be the same thing, so long as the whole formula is surrounded by parenthesis? thanks for any help
 
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2XY means two moles of the species XY. (XY)2 means one mole of the species (XY)2, which contains two XY units bound together. for example, (CN)2 is the molecule cyanogen. We could write it as C2N2, but writing it as (CN)2 emphasises that it is made up of two CN groups joined together, as N≡C-C≡N, rather than any other arrangement of two C and two N atoms. 2CN would imply two separate CN radicals.
 
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instantresults said:
What exactly is the difference between something like (XY)2 and 2XY? I get that the 2XY would indicate 2 moles of XY, but I don't see why (XY)2 wouldn't be the same thing, so long as the whole formula is surrounded by parenthesis? thanks for any help
When a coefficient is placed in front of a molecule, that coefficient will show how many moles are present of the entire molecule following it.

When a subscript is placed after an atom or the parentheses of a molecule, that indicates how many moles of that specific atom or molecule (in parentheses) that is represents.

Let's examine Aluminum Carbonate...

5 Al2(CO3)3

The coefficient 5 represents that there will be 5 moles of Al2(CO3)3 molecules present.
The subscript 2 after Aluminum represents that for every molecule of Aluminum Carbonate, there will be 2 Al atoms present.
The subscript 3 after Oxygen represents that for every molecule of Carbonate, there will be 3 O atoms present.
The subscript 3 after Carbonate represents that for every molecule of Aluminum Carbonate, there will be 3 CO3 molecules present.

Also, to find the number of moles of each individual atom/molecule, multiply the leading coefficient by all subscripts that apply to it. (ex: in the previous example there are 45 moles of Oxygen, found by multiplying coefficient 5 by subscripts 3 and 3)

mjc123 said:
(XY)2 means one mole of the species (XY)2, which contains two XY units bound together.
If it is a standalone molecule then yes. If there is another atom(s) or molecule(s) involved then they could each be individually bound to that other atom or molecule.