High School Chemistry : Gases and Gas Laws

Study concepts, example questions & explanations for High School Chemistry

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Example Questions

Example Question #1 : Using Boyle's Law

A gas in a \(\displaystyle 30.0\, mL\) container is at \(\displaystyle 1.05\, atm\) is compressed to a volume of \(\displaystyle 15.0\, mL\). What is the new pressure of the container?

Possible Answers:

\(\displaystyle P_{2}=0.525\, atm\)

\(\displaystyle P_{2}=472.5\, atm\)

\(\displaystyle P_{2}=2.10\, atm\)

\(\displaystyle P_{2}=1.05\, atm\)

Correct answer:

\(\displaystyle P_{2}=2.10\, atm\)

Explanation:

Boyle's Law is:

\(\displaystyle P_{1} V_{1}=P_{2} V_{2}\)

The initial volume (\(\displaystyle V_{1}\)) and pressure (\(\displaystyle P_{1}\)) of the gas is given. The volume changes to a new volume (\(\displaystyle V_{2}\)). Our goal is to find the new pressure (\(\displaystyle P_{2}\)). Solving for the new pressure gives:

\(\displaystyle \frac{P_{1} V_{1}}{V_{2}}=\frac{P_{2} V_{2}}{V_{2}}\rightarrow P_{2}=\frac{P_{1} V_{1}}{V_{2}}\)

\(\displaystyle P_{2}=\frac{(1.05\, atm)(30\, mL)}{15\, mL}=2.10\, atm\)

Notice the answer has 3 significant figures.

Example Question #1 : Using Gay Lussac's Law

What happens to the pressure of a closed gaseous system when the temperature increases two-fold?

Possible Answers:

The pressure is quadrupled

The pressure stays the name

The pressure is halved

The pressure is doubled

The pressure is quartered

Correct answer:

The pressure is doubled

Explanation:

To solve this question, we will need to use Gay-Lussac's Law:

\(\displaystyle \frac{P_1}{T_1}=\frac{P_2}{T_2}\)

This law shows a direct relationship between temperature and pressure: as temperature increases, pressure will increase proportionally. Mathematically, we can solve for the change in pressure by inputting the change in temperature. The second temperature is equal to twice the first temperature. For the fractions to remain equal, the second pressure must also be twice the first pressure.

\(\displaystyle \frac{2}{2}\)\(\displaystyle \frac{P_1}{T_1}=\frac{P_2}{T_2}\)

Example Question #2 : Using Gay Lussac's Law

An ideal gas is contained in a \(\displaystyle \small 2.5L\) container at a pressure of \(\displaystyle \small 4.0atm\). The container is at a temperature of \(\displaystyle 25^{\circ}C\).

What will be the final pressure if the temperature is increased to \(\displaystyle 50^{\circ}C\)?

Possible Answers:

\(\displaystyle 4.3atm\)

\(\displaystyle 2.0atm\)

\(\displaystyle 5.6atm\)

\(\displaystyle 8.0atm\)

Correct answer:

\(\displaystyle 4.3atm\)

Explanation:

Since the only variable that has changed is temperature, we can use Gay-Lussac's law in order to compare pressure to temperature. Because temperature and pressure are on opposite sides of the ideal gas law, they are directly proportional to one another. In other words, as one increases, the other increases as well.

Gay-Lussac's law is written as follows:

\(\displaystyle \frac{P_{1}}{T_{1}} = \frac{P_{2}}{T_{2}}\)

To use this equation, we must convert the temperature to Kelvin.

\(\displaystyle 25^oC+273=298K\)

\(\displaystyle 50^oC+273=323K\)

Now, use these temperatures and the initial pressure to solve for the final pressure.

\(\displaystyle \frac{4atm}{298K} = \frac{P_{2}}{323K}\)

\(\displaystyle P_{2} = 4.3atm\)

Example Question #1 : Using Gay Lussac's Law

What law is represented by the following formula?

\(\displaystyle \frac{P_{1}}{T_{1}}=\frac{P_{2}}{T_{2}^{}}\)

Possible Answers:

Ideal gas law

Charles's law

Combined gas law

Boyle's law

Gay-Lussac's law

Correct answer:

Gay-Lussac's law

Explanation:

Gay-Lussac's law defines the direct relationship between temperature and pressure. When the parameters of a system change, Gay-Lussac's law helps us anticipate the effect the changes have on pressure and temperature.

\(\displaystyle \frac{P_{1}}{T_{1}}=\frac{P_{2}}{T_{2}^{}}\)

Boyle's law relates pressure and volume: \(\displaystyle P_1V_1=P_2V_2\)

Charles's law relates temperature and volume: \(\displaystyle \frac{V_1}{T_1}=\frac{V_2}{T_2}\)

The combined gas law takes Boyle's, Charles's, and Gay-Lussac's law and combines it into one law: \(\displaystyle \frac{P_{1}V_{1}}{T_{1}}=\frac{P_{2} V_{2}}{T_{2}}\)

The ideal gas law relates temperature, pressure, volume, and moles in coordination with the ideal gas constant: \(\displaystyle PV=nRT\)

Example Question #2 : Using Gay Lussac's Law

The graph depicted here represents which of the gas laws?

Gay-lussac_gas_law_graph

Possible Answers:

Charles's law

Newton's second law

Boyle's law

Gay-Lussac's law

Correct answer:

Gay-Lussac's law

Explanation:

The graph shows that gas pressure varies directly with Kelvin temperature at a constant volume, as determined by Gay-Lussac. Gay-Lussac's law can be represented mathematically for a given mass of gas at a constant volume as follows:

\(\displaystyle \frac{P_1}{T_1}=\frac{P_2}{T_2}\)

Boyle's law shows the relationship between pressure and volume. Charles's law shows the relationship between volume and temperature. Newton's second law is not related to gases and shows the relationship between force, mass, and acceleration.

Example Question #2 : Using Gay Lussac's Law

A family leaves for summer vacation by driving on the highway. The car’s tires start the trip with a pressure of \(\displaystyle 3.8atm\) at a temperature of \(\displaystyle 19^oC\) and a volume of \(\displaystyle 1.7L\). What is the pressure of the tires after driving, when the temperature within the tire increases to \(\displaystyle 105^oC\)?  

Possible Answers:

\(\displaystyle 3.6atm\)

\(\displaystyle 4.4atm\)

\(\displaystyle 4.9atm\)

\(\displaystyle 4.1atm\)

Correct answer:

\(\displaystyle 4.9atm\)

Explanation:

We can assume that the volume of the tires remains constant. This allows us to apply Gay-Lussac's law, which related pressure and temperature:

\(\displaystyle \frac{P_1}{T_1}=\frac{P_2}{T_2}\)

We know both the initial and final temperatures, but we must convert to Kelvin in order to use SI units.

\(\displaystyle 19^oC+273=292K\)

\(\displaystyle 105^oC+273=378K\)

Using these temperatures and the initial pressure, we can solve for the final pressure.

\(\displaystyle \frac{3.8atm}{292K}=\frac{P_2}{378K}\)

\(\displaystyle P_2=\frac{(3.8atm)(378K)}{292K}\)

\(\displaystyle P_2=4.9atm\)

Example Question #33 : Phases Of Matter

If the temperature of a gas increases, what happens to the pressure?

Possible Answers:

Pressure will remain unchanged

Pressure will increase

Pressure will decrease

We must know the volume of the gas to determine the relationship

We must know the molecular formula of the gas to determine the relationship

Correct answer:

Pressure will increase

Explanation:

According to Gay-Lussac's law, the attributes of pressure and temperature vary by direct proportionality. As temperature increases, pressure increases as well.

\(\displaystyle \frac{P_1}{T_1}=\frac{P_2}{T_2}\)

Example Question #4 : Ideal Gas Law

If the pressure of a sample of one mole of an ideal gas is increased from 2atm to 3atm at a constant volume, and the initial temperature was 20˚C, what is the final temperature of the sample?

Possible Answers:

30K

195.3K

303K

439.5K

Correct answer:

439.5K

Explanation:

Because the mass and volume of the sample of the ideal gas are kept constant, a change in pressure causes only a direct change in the temperature. This can be derived from the following ideal gas equation:\(\displaystyle PV=nRT\).

\(\displaystyle \frac{T_{2}}{T_{1}}=\frac{P_{2}}{P_{1}}\)

\(\displaystyle 20^oC+273=293K\)

 

\(\displaystyle \frac{T_{final}}{293 K}=\frac{3 atm}{2 atm}\)

\(\displaystyle T_{final}=439.5 K\)

Example Question #32 : Gases

A sample of chlorine gas fills a vessel at a temperature of \(\displaystyle \small 37^oC\). The vessel has a volume of 3L and experiences a pressure of 3atm. What is the mass of the chlorine gas in the vessel?

\(\displaystyle \small R=0.08206\frac{L*atm}{mol*K}\)

Possible Answers:

\(\displaystyle \small 105.1g\)

\(\displaystyle \small 210.2g\)

\(\displaystyle \small 25.1g\)

\(\displaystyle \small 12.5g\)

Correct answer:

\(\displaystyle \small 25.1g\)

Explanation:

This problem requires us to substitute the moles of gas in the ideal gas law to mass over molar mass.

\(\displaystyle PV=nRT\)

\(\displaystyle n=\frac{mass}{molar\ mass}=\frac{m}M{}\)

We can then isolate for the mass of chlorine gas in the vessel.

\(\displaystyle \small PV = \frac{mRT}{M}\)

\(\displaystyle \small \frac{PVM}{RT} = m\)

Chlorine is diatomic, so its molecular weight will be twice the atomic mass.

\(\displaystyle M=2(35.5\frac{g}{mol})=71\frac{g}{mol}\)

The temperature must be expressed in Kelvin.

\(\displaystyle ^oC+273=K\rightarrow 37^oC+273=310K\)

Using these values, we can solve for the mass of chlorine gas in the vessel.

\(\displaystyle \small m = \frac{(3atm)(3L)(71\frac{g}{mol})}{(0.08206)(310K)} = 25.1g\)

 

Example Question #1 : Using Gay Lussac's Law

A closed flask contains gas at STP. If the temperature is raised to \(\displaystyle 400K\) what will the pressure be in mmHg?

Possible Answers:

\(\displaystyle 780\,mmHg\)

\(\displaystyle 519\,mmHg\)

\(\displaystyle 1110\,mmHg\)

\(\displaystyle 144\,mmHg\)

Correct answer:

\(\displaystyle 1110\,mmHg\)

Explanation:

Since we have information about temperature and we are interested in changes in pessure, Gay-Lussac's Law will be used. This says that

\(\displaystyle \frac{P_{1}}{T_{1}}=\frac{P_{2}}{T_{2}}\)

The initial temperature and pressure are given (STP) meaning a temperature of \(\displaystyle 273K\) and a pressure of \(\displaystyle 760\,mmHg\). There are other values for standard pressure but they have different units and since the problems requests an answer in units of \(\displaystyle mmHg\) we must use the value of standard pressure with those units. Solving for the final pressure gives

\(\displaystyle P_{2}=\frac{P_{1}T_{2}}{T_{1}}\)

Plugging in our variables gives

\(\displaystyle P_{2}=\frac{(760\,mmHg)(400K)}{(273K)}=1113.55\,mmHg\)

We want 3 significant figures since the temperature given has 3, so the final answer is

\(\displaystyle P_{2}=1110\,mmHg\)

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