2 short chemistry lab questions

I have 2 chem lab  questions. the first one has tow part and I attached the experiment paper with the questions

1-what co cause the experimental wei to be

(a)lower than expected 

(b)higher than expected (please not same explanation for both)

2-suppos the Mg used in the experiment had a thin oxide coating. how would this affect the ratio of moles of oxygen per mole of Mg (calcu quan in step 8) explain 

I need it in 1 to 2 ho maximum and pleas full answer as short as you can 

please don't si the hanf shak before being sure that you can do it right