### All Physical Chemistry Resources

## Example Questions

### Example Question #1 : Gases

A researcher places a closed piston container at room temperature. He places a Bunsen burner at the bottom of the container and observes that the piston moves up. What can best explain this phenomenon?

**Possible Answers:**

The pressure of the gas decreased because of Boyle’s Law

The volume of the gas increased because of Boyle’s Law

The pressure of the gas decreased because of Charles’ Law

The volume of the gas increased because of Charles’ Law

**Correct answer:**

The volume of the gas increased because of Charles’ Law

There are three main gas laws. Avogadro’s law states that the moles of a gas is directly proportional to the volume (under constant pressure and temperature). Boyle’s law states that the pressure of the gas is inversely proportional to the volume (under constant moles and temperature). This means that the pressure decreases proportionally when the volume increases. Charles’ law states that the temperature is directly proportional to the volume (under constant moles and pressure). The question states that the piston moves up. This means that the volume of the gas is expanding inside and pushing the piston up to make room for the expanding gas. The gas expansion occurs due to increased temperature (Charles’ law).

### Example Question #1 : Gases

At constant temperature and moles, which of the following is true regarding pressure and volume of a gas?

**Possible Answers:**

Pressure and volume have an exponential relationship with a negative correlation

Pressure and volume are independent of each other

Pressure and volume have an exponential relationship with a positive correlation

Pressure and volume have a linear relationship with a positive correlation

**Correct answer:**

Pressure and volume have a linear relationship with a positive correlation

First, we need to figure out which gas law is applicable here. The question states that temperature and moles are constant. This means that we are dealing with Boyle’s law, which states that pressure is inversely proportional to volume. Inversely proportional means that the pressure decreases as volume increases and vice versa. Note that the relationship is still linear (change in one variable causes a proportional change in the other variable), but the two variables have a negative correlation. Positive correlation means increasing or decreasing a variable would also increase or decrease the other variable, respectively.

### Example Question #1 : Gas Laws

According to __________ law, increasing temperature will __________ volume at constant pressure and constant moles.

**Possible Answers:**

Charles’ . . . decrease

Boyle’s . . . increase

Charles’ . . . increase

Boyle’s . . . decrease

**Correct answer:**

Charles’ . . . increase

Recall that Charles’ law defines the relationship between temperature and volume at constant pressure and constant moles. The law states that the two variables are directly proportional to each other. This means that increasing or decreasing temperature will have the same effect on volume. This makes sense because increasing the temperature increases the kinetic energy of the particles. This makes it easier for particles to move away from each other and expand, which results in an increase in volume.

### Example Question #2 : Gases

An unknown gas is being analyzed in lab. You place of this gas in a container at . You observe that the pressure and volume of the gas is and , respectively. What is the identity of the gas? Assume the gas behaves ideally.

**Possible Answers:**

Argon

Chlorine

Oxygen

Nitrogen

**Correct answer:**

Oxygen

To solve this question we need to use the ideal gas law equation:

Above, is pressure in , is volume in liters, is moles, is , and is temperature in Kelvins. The question gives us pressure, volume, and temperature; therefore, we can solve for . First, we need to convert temperature from Celsius to Kelvins.

Rearrange the ideal gas law and solve for :

The question states that you place one gram of gas in the container. Since we know the moles, we can solve for the molecular weight of the gas and figure out its identity from the molecular weight.

The molecular weight of oxygen gas, is ; therefore, the unknown gas must be oxygen.

### Example Question #3 : Gas Laws

A sample of argon gas at a pressure of 0.959atm and a temperature of , occupies a volume of 563mL. If the gas is compressed at constant temperature until its pressure is 1.40atm, what will be the final volume of the sample of gas?

**Possible Answers:**

**Correct answer:**

The argon gas, as the problem states, is under constant temperature with varying volume and pressure, so we need to use Boyle's Law:

We are given the initial pressure, the initial volume, and the final pressure, and need to solve for the final volume. Therefore, we can rearrange Boyle's law to be as follows:

Plug in known values and solve.

### Example Question #4 : Gas Laws

A sample of methane gas at a pressure of 0.723atm and a temperature of , occupies a volume of 16.0L. If the gas is allowed to expand at constant temperature to a volume of 24.2L, what will the pressure of the gas sample be?

**Possible Answers:**

**Correct answer:**

The methane gas, as the problem states, is under constant temperature with varying volume and pressure, so we need to use Boyle's Law:

We are given the initial pressure, the initial volume, and the final volume and need to solve for the final pressure. Therefore, we can rearrange Boyle's law to be as follows:

Plug in known values and solve.

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