What mass of oxygen gas reacted with the magnesium ?

Question

Answer 1

1.10 g O2

You can get to this two ways. 

1) You can subtract the mass of the crucible + Mg from the Mass of the crucible + MgO

2) Calculate the mass of Mg (subtract mass of empty crucible from mass of crucible + Mg)

Then calculate the mass of MgO (Subtract mass of empty crucible from mass of crucible + MgO)

Finally, subtract mass of Mg from mass of MgO