How can the mass of the oxygen that reacted with magnesium be calculated in the following scenario?The mass of the crucible to start with was...

The mass of the empty crucible you started with was 9.295
g. And the mass of the magnesium you used was 0.426
g.

After heating the magnesium in the crucible you weighed
it again and found its mass to be 9.733g

The final mass
measured included the mass of the crucible, the mass of the magnesium you used and the
mass of the oxygen that reacted with the magnesium. This was equal to
9.733g.

The sum of the mass of the crucible and the
magnesium is 9.295 + 0.426 = 9.721 g

But the final mass you
got was 9.733, so the extra mass was due to the oxygen that reacted with the
magnesium.

This is equal to 9.733 - 9.721 = 0.012
g.

The mass of the oxygen that reacted with
magnesium was 0.012g.