Final answer:
The property of methane gas being described as 4.05 atm in a reaction vessel is pressure, which is one of the four fundamental properties of gases along with volume, temperature, and amount.
Step-by-step explanation:
When a chemist fills a reaction vessel with 4.05 atm of methane, the property of the gas being described is pressure. Pressure is one of the four primary physical properties of a gas, along with volume, temperature, and amount (in moles). These properties are foundational to understanding gas behavior and are essential components of the gas laws, including the ideal gas law, which helps in calculating any of the four properties if the other three are known. In the context of methane in the reaction vessel, the pressure is given directly as 4.05 atm, which is a measure of the force that the gas exerts on the walls of its container.