Question

Of these gases, which has the fastest-moving molecules (on average) at a given temperature?

Answer 1

At a given temperature, lighter gases have faster-moving molecules on average.

Step-by-step explanation:

At a given temperature, all gases have the same average kinetic energy (KEavg) for their molecules. However, gases composed of lighter molecules have more high-speed particles and a higher root mean square velocity (Urms), resulting in a speed distribution that peaks at relatively higher speeds.

On the other hand, gases consisting of heavier molecules have more low-speed particles, a lower Urms, and a speed distribution that peaks at relatively lower speeds.

Therefore, the gas with the fastest-moving molecules (on average) at a given temperature would be the one composed of the lightest molecules.

Answer 2

I believe we are choosing between HBr, NO2, and C2H6.

The molar masses of these compound are:

Molar mass of HBr = 80.9119
Molar mass of NO2 = 46.005
Molar mass of C2H6 = 30.0694

The speed of the molecule actually is inversely proportional to its molar mass. Hence, the lighter the molecule, the faster it moves.

Answer:

C2H6 (ethane)