Question

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.

Short answer

The most plausible explanation for the observed pressure drop over time without a change in total mass is either adsorption of the gas molecules to the container walls or a chemical reaction occurring within the container that forms a product with different properties than the initial gas. Further investigation might help to identify the exact cause behind the pressure drop.

Step-by-step solution

Identifying possible causes for the pressure drop

There could be several reasons for the pressure drop in the container without the total mass changing. To find the correct explanation, we have to consider the possibilities.

Check if the gas is undergoing a phase change

When a gas undergoes a phase change (gas to liquid or solid), its pressure will decrease. However, in this case, the temperature is kept constant, so phase change is very unlikely to occur.

Check if the gas is adsorbed by the container walls

Adsorption occurs when gas molecules adhere to the surfaces of a solid container. This can cause the observed pressure drop in the container. The mass of the gas would still appear to be constant because the gas molecules are still present, just adhered to the container surface.

Check if a chemical reaction is occurring

If a chemical reaction occurs within the container, the pressure might drop due to the products of the reaction having different properties than the initial gas. For instance, if the reaction produces a solid or a liquid, the pressure in the container will decrease. Since the mass in the container does not change, this could be a possible reason for the pressure drop over time.

Suggest the most plausible explanation

Based on the analysis, the most plausible explanation for the observed pressure drop over time without a change in total mass is either adsorption of the gas molecules to the container walls or a chemical reaction occurring within the container that forms a product with different properties than the initial gas. Further investigation, such as inspecting the container walls or taking chemical samples from the container, might help to identify the exact cause behind the pressure drop.

Key concepts

Pressure Changes

When the pressure in a container with a known mass of gas begins to drop without an apparent leak, it can be puzzling. It's crucial to differentiate between the possible explanations for this change. Pressure in a gas is highly dependent on the movement of its molecules. When those gas molecules are fewer in number or less active, the pressure decreases.

Therefore, when observing a constant drop in pressure while the total mass remains stable, we must rule out the absence of leaks first. The absence of loss in total mass is a key hint. This observation strengthens the idea that the gas remains in the container, while its activity or interaction with other materials might be causing the observed decrease in pressure. This leads to considerations of other processes that influence pressure without affecting the overall mass.

Adsorption

Adsorption is a process where gas molecules adhere to a surface of a solid. This process can explain a decrease in pressure within a container. When gas molecules adsorb onto the walls of a container, they effectively reduce the number of 'free' gas particles moving around, thus reducing the measured pressure inside.

Several factors affect adsorption:

• Surface Area: More area for the gas to adhere to increases adsorption.
• Molecule Interactions: Attraction between gas molecules and the container surface enhances adsorption.
• Conditions: Temperature, pressure, and the nature of the gas and surface material.

Adsorption does not alter the mass of the container plus its contents, which explains why the scale still shows the same total mass. However, it significantly alters the state in which gas molecules exist within the container, hence affecting the pressure readings.

Chemical Reactions

Chemical reactions within a container can lead to pressure changes, depending on the nature of the reactants and products. In a closed system with a fixed amount of gas, if the gas undergoes a chemical reaction, it could transform into different compounds, impacting pressure.

Consider a scenario where a gas reacts to form a liquid or solid. The solid or liquid occupies less space than the gas and does not contribute to pressure as a gas would. This results in a net decrease in gas molecules available to exert pressure, thereby lowering the pressure inside the container.

Since the mass remains unchanged, it implies no material is escaping. All transformations occur within the confines of the container. Identifying the specific chemical reactions taking place can be challenging but could include observing changes in the physical state of substances inside the container.

Phase Changes

A phase change involves transforming a substance from one state of matter to another, such as from gas to liquid or solid. This process can also impact pressure within a container. However, in the context of our problem where temperature is constant, phase changes are less likely to be the cause of observed pressure changes.

In typical scenarios without external temperature changes, gases do not readily condense to liquids or solids. A critical factor for a phase change at constant temperature would be if the container material or contents have properties that induce such changes.

Given the right conditions (such as sudden extreme pressure changes or supercooling phenomena), such an occurrence might happen, but it requires specific, rare conditions. Phase changes in most cases involve energy transit processes that require a substantial boost or drop in thermal conditions. Therefore, under constant temperature, looking for other reasons such as adsorption or chemical reactions often provides more plausible explanations for a pressure drop.