Why don't we measure mass of oxygen used in the calorimeter


> 
> The apparatus shown in the diagram can be used to measure the enthalpy change of formation of magnesium oxide (MgO).
> [![enter image description here](https://i.stack.imgur.com/pV7Tt.jpg)](https://i.stack.imgur.com/pV7Tt.jpg)
> List the measurements you would need to make using this apparatus in order to calculate the enthalpy change of formation of magnesium oxide (MgO).
> 
> 
> 


And the mark scheme said:



> 
> Measurements needed:
> 
> 
> * volume/mass/weight of water (in the calorimeter)
> * initial + final temperature/temperature change/temperature rise (of the water)
> * mass of Mg (used)/mass MgO
> * (Not volume/moles/mass of oxygen used)
> 
> 
> 


Why shouldn't I measure the mass of oxygen? Oxygen is also the reactant in its standard state as same as the magnesium element, isn't it?


