Gas Laws 2

The content that follows is the substance of lecture 19. In this lecture we cover the Gas Laws: Stoichiometry, Density and Molecular Weight Calculations.

Gas Laws and Stoichiometry:

The use of the ideal or combined gas law in conjunction with stoichiometry connects at the calculation of moles in either case. What is meant by this is you can use a balanced chemical equation to determine the number of moles of gas being produced and then use the ideal gas law to find its pressure or volume etc. Or conversely you can use the ideal gas law to determine the amount of moles of a gas at a certain temperature and pressure and then use those moles in a stoichiometric calculation.

Examples:

 

 

And going the other way, Example 2:

 

 

For a few practice problems try this link or this link

Gas Laws and Density Calculations:

If we remember back to the beginning of the term the first relationship we learned was that of mass and volume to calculate the density of different materials. We have since seen several uses for the density in both conversions (volume to mass or mass to volume) or in practical application (Eg. will it float?) so we will now again introduce the concept of density in relationship to its use in gases.

First let's look at the Ideal Gas Law and see if we can rearrange it somewhat to be able to generate a density:

As you can see we managed to get the moles of the gas over the volume but this is not quite right since density is actually mass over volume. So how do we go from moles to mass? Well, the usual way...use the molecular weight of the gas:

 

Substituting the MW = Mass/Mole relationship into the gas law produces the equation shown above. Now you just need to use it to answer questions.

Here is an example:

And some Practice Problems: Here and Here

 

Molecular Weight of a Gas

It makes sense that if we can calculate the density of a gas using the Ideal Gas Law with some slight rearrangement that we can calculate the molecular weight of a gas in much the same way:

Here is an example calculation:

 

 

And now an example using density:

 

And here are some practice problems: