[RogueRose]

I've been trying to work out how much H2O2 there is per gram of sodium percarbonate.  Basically if I were to dissolve 1g into 10 or 100ml of water, what percent concentration of H2O2 would I have? 

I've found the molecular weight of 2Na2CO3 * 3H@O@ is 313.96g or Na2CO3 * 1.5H2O2 156.98g.  If the carbonate is anhydrous (which I would think it would have to be having the peroxide attached..??) it has a mass of 106g leaving 51g for the H2O2 at 32.5%  of the molecular weight. 

H2O2 has a molar mass of 34.0147 g/mol, IDK if that is needed or not.  It has a density of 1.450 at 100% and decreases as the percentage to water decreases.  35% is 1.13g/ml and 3% is 1.0095g/ml.

If 1g NaCO3*1.5H2O2  is added to 99.5ml of H2O (the percarb is 2.1g/cc, so I subtracted /5ml or water)  the .325g of H2O2 will dissolve into solution giving an unknown concentration.  IDK if I'm making this too complicated or what.

100ml/.325 = .325%  IDK if that gives the correct percentage of H2O2 so that is why I am asking. 

[Hunter2]

To complicated.

You have molecular weight of the Sodiumpercarbonate. From this you can calculate how many moles you dissolve if you use 1 g of it.  This moles you can calculate bag to the mass of H₂O₂.

[AWK]

Which concentration you should calculate - molar or mass percentage?

If 1g NaCO3*1.5H2O2  is added to 99.5ml of H2O (the percarb is 2.1g/cc, so I subtracted /5ml or water)  the .325g of H2O2 will dissolve into solution giving an unknown concentration.  IDK if I'm making this too complicated or what.

.
From this data you can calculate  the percent concentration easily.
Since we do not know a density of the final solution (and solution will be quite diluted) we neglect volume change after dissolution for calculation of the molar concentration.

[Borek]

No need to worry about the density of the final solution - just assume there are not 10 (100) mL of water, but 10 (100) g of water. Then the result is correct by definition.

If 1g NaCO3*1.5H2O2  is added to 99.5ml of H2O (the percarb is 2.1g/cc, so I subtracted /5ml or water)  the .325g of H2O2 will dissolve into solution giving an unknown concentration.

0.325 g of H₂O₂ per 1 g of 2Na₂CO₃·3H₂O₂ is a perfectly correct number.

And yes, your final number (0.325%, w/v in your case, but most likely almost identical with w/w in such a diluted solution) looks reasonably correct as well (even if you got the calculations reversed in the last line of your post).