Why does the nitrogen concentration decrease after hydrogen is added? n2 (g) + 3h2 (g) ⇌2nh3 (g) why does the nitrogen concentration decrease after hydrogen is added?

The chemical reaction of a reversible reaction is actually governed by Le Chatelier’s principle. It states that when more reactants are introduced into the system, the reaction will proceed forward to create more products. So since Hydrogen is a reactant and Nitrogen is also reactant so adding more Hydrogen makes more products hence reducing the Nitrogen concentration.