12.00 moles of naclo3 will produce how many grams of o2? (moles to grams)

2NaClO₃   →  2NaCl  +  3O₂

mole ratio of NaClO₃  to  O₂  is  2  :  3

∴  if moles of NaClO₃  =  12 mol

then moles of O₂  =  
      
                             =  18 mol

Mass of O₂  =  mol of O₂  ×  molar mass of O₂
 
                    =  18 mol  × 16 g/mol
 
                    =  288 g

So I wasn't sure which equation to use since you did not specify so I just used the decomposition reaction.  If you should have used another reaction then just follow the same steps and you'll get your answer.

Yes it would be a physical change.