Would extra mass in the crucible cause the "mass of oxygen" to come out too high or too low? explain.

 Assuming that the experiment started with the magnesium and did not combine with oxygen, this extra mass would actually decrease the amount of oxygen. This is because it only artificially increases the original mass. Therefore this actually increases the error.

too low

periodic trends is an atom that gets larger as the number of electronic shells increase, the radius of atoms increases as you go down a certain group in the periodic table of elements. in general, the size of an atom will decrease as you move from left to right of a certain period.