A 1.0 L sample of a pure gas is found to have a lower pressure than that predicted by the ideal gas — States of Matter Chemistry Question

STEPS:

1. Recall the assumptions of the Kinetic Molecular Theory (KMT) for an ideal gas: Under ideal conditions, gas particles are assumed to have negligible particulate volume and experience zero intermolecular attractive or repulsive forces.
2. Identify how real gases deviate from ideal behavior: In real gases, molecules *do* experience intermolecular attractions. These attractive forces become increasingly significant when molecules are closer together (such as in a high-density, low-temperature, or high-pressure state).
3. Analyze the microscopic impact of intermolecular attractions: When gas molecules are attracted to one another, they pull on neighboring molecules. As a gas molecule travels toward a container wall, the cohesive, attractive forces from surrounding molecules in the bulk gas pull it backward.
4. Relate molecular collisions to macroscopic pressure: Because of these mutual attractive forces, the molecules strike the container walls less frequently and with less force than they would if they had no attractions among themselves. Because gas pressure is a macroscopic measurement of the force exerted per unit area by these wall collisions, the observed pressure of the real gas is lower than predicted by the ideal gas law.
5. Conclude: The observation of a lower-than-expected pressure is best explained by the molecules being attracted to each other, which reduces the force of their collisions with the walls. This corresponds to Option C.

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WHY_OTHERS_WRONG:

• Option A is incorrect: The combined finite volume of the gas molecules (their "molecular volume") reduces the empty space available for the gas molecules to move. This particulate volume effect dominates at extremely high pressures and causes real gases to have a *higher* pressure than predicted by the ideal gas law, not a lower pressure.
• Option B is incorrect: According to kinetic molecular theory, at a given temperature, all gas samples have the same average kinetic energy regardless of their identity. While lighter gas molecules have a lower mass, they travel at higher speeds, which compensates for the lower mass. Therefore, a low molecular mass does not cause a gas to exert less pressure than expected.
• Option D is incorrect: If gas molecules were attracted to the sides of the container and struck them with more force, the observed pressure would be *higher* than predicted, which directly contradicts the experimental finding of a lower pressure.