Final answer:
The rate of change of HCl(g) is -0.024 m/s.
Step-by-step explanation:
The reaction H₂(g) + Cl₂(g) → 2HCl(g) has a rate of change of H₂(g) of -0.012 m/s. Since the stoichiometric coefficient of HCl(g) is 2 in the balanced equation, the rate of change of HCl(g) must be twice the rate of change of H₂(g). Therefore, the rate of change of HCl(g) is -0.024 m/s.