Final answer:
Adding 1.41 mol of H2 gas while keeping the volume and temperature constant does not change the average molecular speed of the system; it d)remains the same.
Step-by-step explanation:
The question asks about the effect of adding 1.41 mol of H2 gas to a system with constant volume and temperature on the average molecular speed of the total system. When more molecules of a gas are added to a container without changing the temperature, the average molecular speed remains the same. This is because the average kinetic energy, which is related to molecular speed, is determined by the temperature according to the equation KEavg = (3/2)kT, where k is the Boltzmann constant and T is the temperature. Since the temperature does not change, neither does the average kinetic energy or the average speed of the molecules.
Therefore, the correct answer is D) remain the same.