Gases

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1

A gas sample is contained in a 4L vessel at a pressure of 3atm. Assuming all other conditions are kept constant, what is the new pressure in the vessel if the volume is reduced to 1.5L?

Explanation

According to Boyle's law, pressure and volume are inversely proprotional to each other. This is represented by the equation:

In other words, as volume decreases in a vessel, the pressure will increase, and vice versa. Using the given conditions, we can solve for the final pressure in the vessel:

2

A gas sample is contained in a 4L vessel at a pressure of 3atm. Assuming all other conditions are kept constant, what is the new pressure in the vessel if the volume is reduced to 1.5L?

Explanation

According to Boyle's law, pressure and volume are inversely proprotional to each other. This is represented by the equation:

In other words, as volume decreases in a vessel, the pressure will increase, and vice versa. Using the given conditions, we can solve for the final pressure in the vessel:

3

Which of the following is relevant for real gases, but irrelevant for ideal gases?

I. Volume of gas particles

II. Intermolecular forces between gas particles

III. Volume of container

I and II

I only

III only

I and III

Explanation

There are two main assumptions for an ideal gas (and a few smaller assumptions). First, the gas particles of the ideal gas must have no molecular volume. Second, the gas particles must exert no intermolecular forces on each other; therefore, forces such hydrogen bonding, dipole-dipole interactions, and London dispersion forces are irrelevant in ideal gases. Other small assumptions of ideal gases include random particle motion (no currents), lack of intermolecular interaction with the container walls, and completely elastic collisions (a corollary of zero intermolecular forces).

For real gases, however, these assumptions are invalid. This means that the real gas particles have molecular volume and exert intermolecular forces on each other.

Recall that the volume in the ideal gas law is the volume of the free space available inside the container. For ideal gases, the free space volume is equal to the volume of the container because the gas particles take up no volume; however, for real gases, the free space volume is the volume of the container minus the volume of the gas particles. Though the exact values of free space volume will differ, the volume of the container is important for both real and ideal gases.

4

Which of the following is relevant for real gases, but irrelevant for ideal gases?

I. Volume of gas particles

II. Intermolecular forces between gas particles

III. Volume of container

I and II

I only

III only

I and III

Explanation

There are two main assumptions for an ideal gas (and a few smaller assumptions). First, the gas particles of the ideal gas must have no molecular volume. Second, the gas particles must exert no intermolecular forces on each other; therefore, forces such hydrogen bonding, dipole-dipole interactions, and London dispersion forces are irrelevant in ideal gases. Other small assumptions of ideal gases include random particle motion (no currents), lack of intermolecular interaction with the container walls, and completely elastic collisions (a corollary of zero intermolecular forces).

For real gases, however, these assumptions are invalid. This means that the real gas particles have molecular volume and exert intermolecular forces on each other.

Recall that the volume in the ideal gas law is the volume of the free space available inside the container. For ideal gases, the free space volume is equal to the volume of the container because the gas particles take up no volume; however, for real gases, the free space volume is the volume of the container minus the volume of the gas particles. Though the exact values of free space volume will differ, the volume of the container is important for both real and ideal gases.

5

Which gas follows the exact definition of the ideal gas law?

None of these

Explanation

Though the ideal gas law gives a nearly close to real approximation of numbers, it oversimplifies its description of gases. No real gas follows the exact definition of the ideal gas law and is very complex because there are intermolecular forces that must be considered. An ideal gas described as a point mass in which the particles are so small that its volume is negligible. However, real gases have real volume. Also, ideal gases are considered elastic, having no attractive and repulsive forces with no energy transfer during collisions. Real gases actually collide and are non-elastic. Note that gases approach ideal behavior as their temperature increases and their pressure decreases.

6

Which gas follows the exact definition of the ideal gas law?

None of these

Explanation

Though the ideal gas law gives a nearly close to real approximation of numbers, it oversimplifies its description of gases. No real gas follows the exact definition of the ideal gas law and is very complex because there are intermolecular forces that must be considered. An ideal gas described as a point mass in which the particles are so small that its volume is negligible. However, real gases have real volume. Also, ideal gases are considered elastic, having no attractive and repulsive forces with no energy transfer during collisions. Real gases actually collide and are non-elastic. Note that gases approach ideal behavior as their temperature increases and their pressure decreases.

7

Which of the following assumptions is not made by the ideal gas law?

The intermolecular interactions follow the Coulomb model of electric repulsion

The size of the molecules is much smaller than the container

The molecules move randomly

The molecules obey Newton's laws of motion at all times

The van der Waals forces are negligible

Explanation

Under the ideal gas law, we assume that the interactions between the molecules are very brief and that the forces involved are negligible. The assumption that the molecules obey Coulomb's law when interacting with each other is not necessary; rather, an ideal gas must disregard Coulomb's law.

The ideal gas law assumes only Newtonian mechanics, disregarding any intermolecular or electromagnetic forces.

8

Which of the following assumptions is not made by the ideal gas law?

The intermolecular interactions follow the Coulomb model of electric repulsion

The size of the molecules is much smaller than the container

The molecules move randomly

The molecules obey Newton's laws of motion at all times

The van der Waals forces are negligible

Explanation

Under the ideal gas law, we assume that the interactions between the molecules are very brief and that the forces involved are negligible. The assumption that the molecules obey Coulomb's law when interacting with each other is not necessary; rather, an ideal gas must disregard Coulomb's law.

The ideal gas law assumes only Newtonian mechanics, disregarding any intermolecular or electromagnetic forces.

9

An unknown amount of neon gas is contained in a 3.00L vessel. At a temperature of , the gas exerts a pressure of 4.00atm.

Neon gas has a molar mass of .

Based on these conditions, what is the mass of neon gas in the vessel?

Explanation

This question deals with the amount of gas present in a vessel for only one set of conditions. This makes the ideal gas law a suitable equation to use in order to determine the amount of gas in the vessel. The ideal gas law is written as:

Using this equation, we can solve for the molar quantity of gas in the vessel:

Knowing this, we can now solve for the mass of the gas in the vessel by multiplying this molar amount by the molar mass:

10

An unknown amount of neon gas is contained in a 3.00L vessel. At a temperature of , the gas exerts a pressure of 4.00atm.

Neon gas has a molar mass of .

Based on these conditions, what is the mass of neon gas in the vessel?

Explanation

This question deals with the amount of gas present in a vessel for only one set of conditions. This makes the ideal gas law a suitable equation to use in order to determine the amount of gas in the vessel. The ideal gas law is written as:

Using this equation, we can solve for the molar quantity of gas in the vessel:

Knowing this, we can now solve for the mass of the gas in the vessel by multiplying this molar amount by the molar mass:

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