You have an evacuated container of fixed volume and known mass and introduce a known mass of a gas sample. Measuring the pressure at constant temperature over time, you are surprised to see it slowly dropping. You measure the mass of the gas-filled container and find that the mass is what it should be—gas plus container—and the mass does not change over time, so you do not have a leak. Suggest an explanation for your observations.

Verified step by step guidance

1. Consider the possibility of gas adsorption onto the container walls, which would reduce the number of gas molecules in the gas phase, leading to a decrease in pressure.
2. Think about the potential for a chemical reaction occurring within the container that consumes gas molecules, thus reducing the pressure.
3. Evaluate whether the gas could be undergoing a phase change, such as condensation, which would decrease the pressure as gas molecules transition to a liquid phase.
4. Examine the temperature stability to ensure that there are no fluctuations causing changes in gas behavior, even though the problem states constant temperature.
5. Reflect on the accuracy and calibration of the pressure measurement device to rule out any instrumental errors that might falsely indicate a pressure drop.

Key Concepts

Ideal Gas Law

The Ideal Gas Law relates the pressure, volume, temperature, and number of moles of a gas through the equation PV = nRT. In this scenario, the pressure drop at constant temperature suggests that the number of gas molecules (n) may be decreasing, which could indicate a change in the gas's state or behavior, rather than a leak.

Gas Behavior and Kinetic Molecular Theory

Kinetic Molecular Theory explains how gas particles behave, emphasizing that gas pressure results from collisions of particles with the container walls. If the pressure is dropping while the mass remains constant, it may imply that the gas is undergoing a phase change or chemical reaction that alters its effective pressure without losing mass.

Real Gases vs. Ideal Gases

Real gases deviate from ideal behavior under certain conditions, such as high pressure or low temperature. If the gas sample behaves non-ideally, interactions between gas molecules could lead to a decrease in pressure over time, even if the mass remains unchanged, due to factors like condensation or adsorption onto the container walls.