Final answer:
The primary factor influencing the behavior of gas in a vertical frictionless piston-cylinder device is temperature, which affects the kinetic energy of gas particles and thus their pressure and volume.
Step-by-step explanation:
When examining the behavior of a gas contained within a vertical frictionless piston-cylinder device, the primary factor influencing the behavior of the gas is the temperature. According to the kinetic-molecular theory, gas particles exert pressure on the walls of their container through collisions that result from their motion. Changes in temperature directly affect the average kinetic energy of gas molecules, which in turn changes their pressure and volume. An increase in temperature, with a constant amount of gas and pressure, results in an increase in volume
Therefore, while pressure, volume, and the composition of the gas all play roles in the gas behavior, temperature is the variable that directly impacts the kinetic energy and thus the initial activity of the gas particles, leading to changes in the other properties.