How can I follow the concentration of nitrogen in the reaction mixture? What's the principle?

Question

This gaseous reaction takes place at (750+273)K in a closed container with a fixed volume.

2H2(g) + 2 NO2(g) = 2H2O(l) + N2(g)

Answer 1

You could put a pressure gauge on the closed container. Since the other product is water as a liquid, the formation of N2 gas will lead to an increase in pressure.

The more gas is produced, the higher the pressure in a fixed volume at a fixed temp.