Physics
posted by ana .
How many atoms of hydrogen are in 220g of hydrogen peroxide (H2O2)?
the molecular weight of H2O2 = 2*1 +2*16 =34
therefore, there are 220/34 moles of the molecule = 6.47 moles, so there are 6.47xAvogadro's number of molecules
there are 2 H atoms per molecule, so multiply 2x6.47xNa to get the final answer (Na=Avogadros Number)...
but for some reason it isn't given the right answer

I agree with your method. Keep four significant figures and round to three when you are done. You might want to use 1.008 for the H atomic weight, but it won't make a lot of difference.