Haha.. I just found a short cut.. You know that 1 mole = 22.4L at STP. Find out how many moles (n) are equal to 250mL. (don't forget to change to L). You will again find that n<1 and is the same fraction (to 5 decimal places) you would get using the Ideal Gas Law. (you might want to do it both ways as a double check)
Then do the same as I described before, multiply the molecular masses of each of your oxides by this fraction and compare to the mass you were given. One of them will agree.
