### All AP Physics 2 Resources

## Example Questions

### Example Question #1 : Properties Of Ideal Gases

Which of the following is not a property of an ideal gas?

**Possible Answers:**

The gas particles have no volume

The gas particles have elastic collisions

The gas particles have strong intermolecular forces acting on them

All of these answers are properties of ideal gases

The gas particles have almost no mass

**Correct answer:**

The gas particles have strong intermolecular forces acting on them

Ideal gases have particles that are considered to be point masses. This means that all of their (extremely small) mass is contained in a single, infinitesimal point of zero volume. These particles experience elastic collisions, which means they lose no kinetic energy when they collide with other particles. If the particles experienced strong intermolecular forces, then they couldn't have elastic collisions, so they wouldn't be ideal gases anymore. Ideal gases don't experience any intermolecular forces.

### Example Question #2 : Properties Of Ideal Gases

A real gas becomes more like an ideal gas at __________ temperatures and __________ pressures.

**Possible Answers:**

lower . . . higher

Real gases can never be made to act more like ideal gases

higher . . . higher

lower . . . lower

higher . . . lower

**Correct answer:**

higher . . . lower

When the temperature is higher, the kinetic energy of particles is higher. Because their energy is higher, they bounce more energetically, letting them ignore most intermolecular forces (which is something ideal gases don't experience).

An important assumption for ideal gases is that the volume of the particles is negligible compared to the volume of their container. For real gases, as pressure increases, the particles get closer together, and their volume gets less negligible when compared to the volume of the container. This is why real gases behave more ideally at lower pressures.

### Example Question #1 : Properties Of Ideal Gases

What is the temperature in Kelvin for 1mol of gas at 5atm and a volume of 10L?

**Possible Answers:**

**Correct answer:**

Use the ideal gas law equation.

Manipulate the equation, substitute the givens, and solve for temperature. Write the gas constants necessary to solve the problem.

### Example Question #1 : Properties Of Ideal Gases

Which of the following is the correct formula to find the internal energy of an ideal gas?

**Possible Answers:**

**Correct answer:**

An ideal gas has no molecular interactions besides perfectly elastic collisions. The energy is the combined kinetic energies of ideal gas molecules since there is no potential energy. For point molecules of the monatomic gas, the equation can be written as:

Note that is the equation for average kinetic energy also in Joules.

### Example Question #1 : Properties Of Ideal Gases

We have of oxygen gas at . We increase the temperature to and keep the pressure the same. What is the new volume?

**Possible Answers:**

**Correct answer:**

We will use the relationship:

Rearrange for the final volume:

Convert our temperatures to kelvin.

Plug in our values and solve.

### Example Question #1 : Properties Of Ideal Gases

We have of gas at . We increase the temperature to , while keeping the pressure constant. What is the new volume?

**Possible Answers:**

**Correct answer:**

We will use the relationship:

Rearrange to solve for the final volume:

Convert our temperatures to kelvin:

Plug in our values and solve.

### Example Question #1 : Properties Of Ideal Gases

We have of helium gas at . We decrease the pressure to , while keeping the temperature the same. What is the new volume?

**Possible Answers:**

**Correct answer:**

We will use the relationship:

Rearrange to solve for the final volume:

Plug in our values and solve.

### Example Question #2 : Properties Of Ideal Gases

We have of neon gas at . We increase the pressure to while keeping the temperature constant. What is the new volume?

**Possible Answers:**

**Correct answer:**

We will use the relationship:

Rearrange to solve for the final volume:

Plug in our values and solve.

### Example Question #1 : Properties Of Ideal Gases

We have of neon gas. We increase the temperature from to , and increase the pressure from to . What will be the new volume?

**Possible Answers:**

**Correct answer:**

First, we will convert celsius to Kelvin.

Then, we will use the relationship:

Solve for volume final volume:

Plug in our values and solve.

### Example Question #1 : Properties Of Ideal Gases

Under which of the following conditions would a gas be expected to deviate the least from ideal behavior?

**Possible Answers:**

High pressure and low temperature

Low pressure and low temperature

High pressure and high temperature

Low pressure and high temperature

**Correct answer:**

Low pressure and high temperature

When talking about gases, it's convenient to talk about gases that behave ideally. The ideal gas law, , generally only holds true for gases that behave ideally. Once gases begin to deviate from ideal behavior, this equation begins to lose its accuracy in its power to predict.

There are a number of assumptions that are made when referring to gases that behave ideally, and these assumptions constitute what is known as the kinetic molecular theory of gases. In this model, there are a total of 5 assumptions:

1. Individual gas particles are very tiny points of mass that essentially have no volume (i.e. the volume of a gas is equal to the volume of free space in the container).

2. These gas particles are neither attracted to, nor repelled by, other gas particles (i.e. there are no intermolecular forces acting between gas particles).

3. Each of these gas particles are continuously moving in random directions.

4. All of these gas particles collide with each other, such that no energy is gained or lost by each collision (i.e. each collision is completely elastic).

5. At a given temperature, the average kinetic energy for a given sample of gas is the same as any gas (i.e. the identity of the gas does not matter) and the average kinetic energy of the sample of gas is directly related to the absolute temperature.

Now that we know what the assumptions are, let's go ahead and see how pressure and temperature affect ideal behavior, since these two parameters show up in each of the answer choices.

**Temperature**

When the temperature of a gas is lowered, the average kinetic energy of the sample of gas also decreases. Consequently, each gas particle making up the solution is expected to slow down. When this happens, the gas particles become more available for interacting with other gas particles, which increases the likelihood for intermolecular forces to begin playing a role. This violates one of the assumptions of kinetic molecular theory. When individual gas particles begin to attract one another, the pressure that the gas exerts on the wall of its container becomes less than what the ideal gas law equation would predict.

Thus, we would expect a high temperature to cause the least deviation from ideal behavior. In other words, a high temperature would increase the average kinetic energy of the sample of gas, and when all of the gas particles are shooting around at greater speeds, they have less of an opportunity to interact with other gas particles.

**Pressure**

When the pressure of a gas is increased, we also see deviations from ideal behavior. Remember that one of the assumptions of kinetic molecular theory is that the individual gas particles are very tiny points of mass that essentially have no volume, which means that the volume of the gas as predicted by the ideal gas law is essentially equal to the volume of the container. But, we of course know that real gases do not have zero volume and instead, each individual gas particle takes up a small amount of volume. As the external pressure becomes greater and greater, the volume of the container decreases as well. This, in turn, causes each gas particle to comprise a greater fraction of the container. Consequently, at high enough pressures, a real gas will take up a significant fraction of the container's volume, meaning that the volume of the real gas will be greater than what is predicted by the ideal gas law.

When external pressures are kept low, the volume of the container is sufficiently large enough that the volume of gas takes up a negligible fraction of the container. Thus, low external pressures will be expected to cause the least deviation from the ideal gas law.

In summary, it's important to remember the following:

- At high pressures, the actual volume of gas particles makes the real volume higher than the predicted volume.
- At low temperatures, intermolecular forces make the actual pressure lower than the predicted pressure.