### All High School Chemistry Resources

## Example Questions

### Example Question #1 : Using The Ideal Gas Law And Combined Gas Law

Which of the following is a condition of the ideal gas law?

**Possible Answers:**

The gas molecules must interact with each other

The gas molecules must be moving at a slow speed

The gas molecules do not have volume

The gas molecules must be in specific mole ratios

The gas molecules must adhere to the container wall

**Correct answer:**

The gas molecules do not have volume

The ideal gas law has some conditions that must be met, conditions that certainly cannot be met in the real world. These conditions include that the gases cannot interact with one another, gases must be moving in a random straight-line fashion, gas molecules must not take up any space, and gases must be in perfect elastic collisions with the walls of the container. These conditions minimize the effect that gas molecules have on one other, allowing a prediction based on completely random and unimpeded molecular movement. In reality, these conditions are impossible. All real gas molecules have a molecular volume and some degree of intermolecular attraction forces.

### Example Question #1 : Using The Ideal Gas Law And Combined Gas Law

A container of gas has a pressure of at a temperature of . The container is expanded to , and the temperature is increased to .

What is the final pressure of the container?

**Possible Answers:**

**Correct answer:**

In this case, two variables are changed between the initial and final containers: volume and temperature. Since we are looking for the final pressure on the container, we can use the combined gas law in order to solve for the final pressure:

When using the ideal gas law, remember that temperature must be in Kelvin, not Celsius, so we will need to convert.

Use the given values to solve for the final pressure.

### Example Question #1 : Using The Ideal Gas Law And Combined Gas Law

At STP, an unknown gas has a density of .

Based on this information and the periodic table, what is the identity of the gas in the container?

**Possible Answers:**

Krypton

Fluorine gas

Nitrogen gas

Neon

**Correct answer:**

Fluorine gas

Once again, we can use the ideal gas law in order to solve for the unknown gas. We will start again with the ideal gas law:

Density has the units of mass over volume, which means that we can rearrange the ideal gas law so that density is on one side of the equation. However, we need to substitute one of the values so mass is also in the equation. Remember that moles are equal to mass divided by molar mass, so we can rewrite "n" in order to solve for density:

STP means that the container is at 1 atmosphere of pressure and 273 Kelvin. Knowing this, we can solve for molar mass:

Fluorine gas has a molar mass of 38.00 grams per mol. As a result, we determine that fluorine gas is the unknown gas in the container.

### Example Question #3 : Using The Ideal Gas Law And Combined Gas Law

of an unknown gas are contained in a container. The container has a pressure of at a temperature of .

Based on this information and the periodic table, which of the following gases is in the container?

**Possible Answers:**

Nitrogen gas

Neon

Helium

Argon

**Correct answer:**

Argon

In order to determine the gas being contained, we are going to need to rewrite the ideal gas law. The ideal gas law is written as follows:

The number of moles of a gas can be rewritten as the mass of the gas divided by its molar mass. Knowing this, we can rewrite the equation, and solve for the molar mass of the gas.

Use the given values in this equation to solve for the molar mass. Remember to first convert degree Celsius to Kelvin!

Now that we have solved for the molar mass, we can see which gas has this molar mass on the periodic table. Argon has a molar mass of 39.95 grams per mole, so we can determine that argon is the gas in the container.

### Example Question #1 : Using The Ideal Gas Law And Combined Gas Law

Which law is represented by the following formula?

**Possible Answers:**

Dalton's law

Charles's law

Ideal gas law

Boyle's law

Combined gas law

**Correct answer:**

Combined gas law

The combined gas law takes Boyle's, Charles's, and Gay-Lussac's law and combines it into one law.

It is able to relate temperature, pressure, and volume of one system when the parameters for any of the three change.

Boyle's law relates pressure and volume:

Charles's law relates temperature and volume:

Gay-Lussac's law relates temperature and pressure:

The ideal gas law relates temperature, pressure, volume, and moles in coordination with the ideal gas constant:

### Example Question #1 : Using The Ideal Gas Law And Combined Gas Law

A scuba diver uses compressed air to breath under water. He starts with an air volume of at sea level () at a temperature of . What is the volume of air in his tank at a depth of () and a temperature of ?

**Possible Answers:**

**Correct answer:**

This question requires the combined equation of the individual gas laws:

To use this equation, we first need to convert the given temperatures to Kelvin.

We now know the initial pressure, volume, and temperature, allowing us to solve the left side of the equation.

Use the given values for the final temperature and pressure to solve for the final volume.

### Example Question #1 : Using The Ideal Gas Law And Combined Gas Law

A sample of helium gas at and a pressure of is used to inflate a balloon. What is the volume this gas occupies when the temperature reaches at a pressure of during the inflation?

**Possible Answers:**

**Correct answer:**

This question requires s to use the combined gas law:

We know the initial pressure, temperature, and volume, allowing us to solve for the left side of the equation.

We are also given the final pressure and temperature. Using these values on the right side of the equation we can solve for the final volume.

### Example Question #1 : Using The Ideal Gas Law And Combined Gas Law

Below is the ideal gas law.

What does each letter of the ideal gas law represent?

**Possible Answers:**

P: potential energy

V: velocity

n: moles

R: ideal gas constant

T: total energy

P: pressure

V: volume

n: moles

R: ideal gas constant

T: temperature

P: pressure

V: volume

n: moles

R: rate

T: time

P: pressure

V: volume

n: moles

R: ideal gas constant

T: time

None of these

**Correct answer:**

P: pressure

V: volume

n: moles

R: ideal gas constant

T: temperature

The ideal gas law is used to identify values in a given state (for the values of pressure, volume, number of moles, and temperature) for an ideal, hypothetical gas. Because no gases are truly ideal, this only works as an approximation, and some gases are more ideal than others.

### Example Question #1 : Using The Ideal Gas Law And Combined Gas Law

A gas in a container is at STP. How many moles of gas are in the container?

**Possible Answers:**

**Correct answer:**

First we need to remember what STP means since it's important. This is "standard temperature and pressure". Because we are looking for the number of moles we need the ideal gas law.

At STP we will take the temperature to be and the pressure to be . Solving for the number of moles gives

Notice that the units of the gas constant R mean we must have a volume in liters. The volume was given in mL and therefore had to be converted.

### Example Question #1 : Using The Ideal Gas Law And Combined Gas Law

A gas at STP has a volume of . The gas is compressed to and heated to . What is the new pressure of the gas in atm?

**Possible Answers:**

**Correct answer:**

We have a lot going on in this problem. Since both temperature and volume change and we want to know the final pressure, the combined gas law will be used:

Since the gas is at STP to start we know the pressure () and the temperature (). Standard pressure has multiple values depending on the units, but we want pressure in atm for our final answer so we will choose the appropriate value and units for STP. Solving for the final pressure gives:

Plugging in everything gives:

We expect that raising the temperature will increase the pressure as well as reducing the volume. Since both actions will increase the pressure we must have a final pressure greater than the initial pressure.

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