[msandra]

From the experiment, i used 0.490g metal (Mg) to combust with Oxygen to form a product 0.912g.

1. when i calculate the % oxygen,
0.422/0.912 x 100%=46.3% is that correct?

2. Then i caluclate the mole of oxygen
46.3g/16.00 = 2.89 mol
Should I use 16 or 32?

[jdurg]

I would assume that you would use 32 since the question is probably asking about how many moles of Oxygen gas are produced.  (However, you could stick with using 16 and just mention that you are reporting monatmoic Oxygen.  If you use 32, just be sure to report it as molecular oxygen gas so that if there are any questions the person going over your work will know what you were using).  

[ssssss]

From the experiment, i used 0.490g metal (Mg) to combust with Oxygen to form a product 0.912g.

1. when i calculate the % oxygen,
0.422/0.912 x 100%=46.3% is that correct?

2. Then i caluclate the mole of oxygen
46.3g/16.00 = 2.89 mol
Should I use 16 or 32?

In the first part you are calculating % of oxygen[ in atoms].So you are using its atomic mass,Whereas in second part where you are dealing with O2 in chemical reaction it means you are considering O2 molecule so you will use its molecular weight[ AND NOT ATOMIC WEIGHT] which is 32.You get it?

[AWK]

See:
http://forums.about.com/n/mb/message.asp?webtag=ab-chemistry&msg=3040.2