For every two moles of Pb(NO3)2, you produce one mole of O2. So, how many moles if 305.80 g? (Hint: Divide by the molecular weight of Pb(NO3)2)
Now remember to correct our the stoichiometry (divide by 2).
Assuming that you wind up with STP conditions (273K and 1 atmosphere), each mole represents 22.4 L -- so multiply the moles by 22.4 and you answer will be in L.