College Chemistry : Gases

Study concepts, example questions & explanations for College Chemistry

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Example Questions

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Example Question #1 : Gases

A gas does  of work while expanding and absorbs  of heat. What is the change in internal energy ?

Possible Answers:

Correct answer:

Explanation:

The correct answer is .

 where  represents heat and  represents work.  

If a gas is expanding, it is doing work on the surroundings.  This means that  will be a negative value. Because the gas is absorbing heat,  entering the system and will be a positive value.  

Example Question #2 : Gases

The graph shows molecular velocities for four different molecules at the same temperature. Which molecule has the highest molar mass?

1

Possible Answers:

B

C

A

D

Correct answer:

A

Explanation:

Recall that the kinetic theory of gases states that if molecules are at the same temperature, then the mass of the molecules will determine their velocities. In a mixture of gases at a given temperature, the heavier gases will, on average, travel slower than the lighter ones. 

Thus, A must be the gas with the highest molar mass because it has the most molecules traveling at the slowest velocity.

Example Question #3 : Gases

Which is not characteristic of gases?

Possible Answers:

Gases expand to fill their containers.

Gases are easy to compress.

The pressure of a gas decreases as more gas is added to the container.

Gases occupy more space than the liquids or solids that they form.

Correct answer:

The pressure of a gas decreases as more gas is added to the container.

Explanation:

There is more free space between gas particles than there is between liquid particles, and more free space between liquid particles than there is between solid particles. This makes gases very easy to compress as compared to solids and liquids.

Both liquids and gases can expand to fill their containers because the individual particles can move past each other. This is not possible in a solid because the individual particles are rigidly packed.

Additionally, the volume of a liquid or solid can increase by approximately eight hundred times when it becomes a gas. This large change in volume can be harnessed to do work. For example, a steam engine works when water boils to form a gas (steam), which has a larger volume. Steam can thus escape from the container in which it was produced and perform work.

Lastly, the pressure of a gas increases as more gas is added to the container. Pressure is defined as the force exerted by the gas over a certain area. If more gas particles are added to a fixed area, they will exert a greater force over that area, increasing the overall pressure.

Example Question #4 : Gases

The Haber process is a common reaction used in the industrial production of ammonia:

Which of the following would increase the production of NH3(g)?

Possible Answers:

Decreasing volume

Adding water

Adding an inert gas

Increasing temperature

Increasing volume

Correct answer:

Decreasing volume

Explanation:

Let's first look at the affects of adding an inert gas such as . He would not take part in the reaction, so the equation can be written as

Since He is on both sides of the equation, equilibrium is not affected.

Now let's look at the affects of heat. Because the reaction is exothermic, it can be rewritten as

Since heat is a product of the reaction, increasing temperature would shift the reaction to the left.

Finally, we know that changing the volume changes the internal pressure of the system. We know that the affect of this change is dependent on the number of moles in the gaseous phase. 

In order to increase the production of NH3 (the side with the least number of moles), we need to increase the internal pressure of the system by decreasing the volume.

Decreasing the volume would increase the production of ammonia.

Example Question #5 : Gases

One flask is at STP and another is at .  What is the pressure at ?

Possible Answers:

Correct answer:

Explanation:

The pressure of the flask at  is .  

Because the volume between the flasks and the moles in each flask are constant, we can cancel out  and .

At STP, conditions are  and .  

Example Question #6 : Gases

A 3.00 L container at 273 K is filled with 1.00 mol Cl2(g), which behaves non-ideally.

Using the van der Waals equation, calculate pressure exerted by the gas.

 

Possible Answers:

Correct answer:

Explanation:

Recall the van der Waals equation for non-ideal gases

Rewrite the equation using the known values and solve for P

Example Question #7 : Gases

What is the pressure of a  cylinder filled with  moles of helium gas at ?

Possible Answers:

Correct answer:

Explanation:

Recall the ideal gas law:

,

where ,

 ,

,

and .

Since the question asks for pressure, rearrange the equation to solve for .

Plug in the given values. Remember that 

Example Question #8 : Gases

A sample of gas at a constant temperature has an initial pressure of  at a pressure of . If the volume of gas is decreased to , what is its pressure?

Possible Answers:

Correct answer:

Explanation:

Since we are given the volume and the pressure of this sample of gas, we will need to use Boyle's Law, which states that the pressure and volume of a gas, at a constant temperature, are inversely related. As thus, we can then write the following equation:

Since all the answer choices are in units of atmospheres, we will need to convert the given units into atmospheres.

Plug in the given pressures and volume into the equation, and solve for .

Example Question #9 : Gases

Consider the following chemical reaction:

How many grams of lithium is needed to react with  of nitrogen gas measured at  and ?

Possible Answers:

Correct answer:

Explanation:

Start by finding the number of moles of nitrogen gas by using the ideal gas law:

Rearrange the equation to solve for the variable :

In order to use the ideal gas law, the pressure must have units of atmospheres and the temperature must have units of Kelvin.

Convert the given pressure into atmospheres:

Now, substitute in all the given values in order to find the number of moles of nitrogen gas.

Next, use the stoichiometric ratio given by the chemical equation to find the number of moles of lithium needed to react completely with the nitrogen gas.

Finally, convert the number of moles of lithium to number of grams of lithium.

 

 

Example Question #10 : Gases

What is the pressure exerted by  in a  container at ?

Possible Answers:

Correct answer:

Explanation:

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