Which of the following gaseous reactions, carried out at constant volume and temperature, cannot be followed by measuring the total pressure?

A $H X_{2} (g) + I X_{2} (g) 2 HI (g)$
B $PCl X_{5} (g) PCl X_{3} (g) + Cl X_{2} (g)$
C $2 NH X_{3} (g) N X_{2} (g) + 3 H X_{2} (g)$
D $2 N X_{2} O X_{5} (g) 4 NO X_{2} (g) + O X_{2} (g)$

Answer: A

Pressure monitoring needs a change in the total number of moles of gas. The HI reaction has 2 mol of gas on each side, so the total pressure stays constant, while the other three reactions all increase the number of moles.

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