Converting from Grams to Molecules
A vat of Hydrogen Peroxide ($\ce{H2O2}$) contains 455 grams of oxygen atoms.
  How many molecules of ($\ce{H2O2}$) are in the vat?



How many moles of oxygen atoms are in 455 gms of oxygen atoms? Let's call the answer $x$.
How many moles of $\ce{O2}$ are in $x$?  Let's call the answer $y$.
How many moles of $\ce{H2O2}$ could you make from $y$ moles of $\ce{O2}$?  Let's call the answer $z$.
1 mole of any substance contains Avogadro's number of molecules.  How many molecules are in $z$ moles?