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Worked example: Calculating partial pressures. Practice: Ideal gas law. Next lesson. Current timeTotal duration Google Classroom Facebook Twitter. Video transcript - [Instructor] In this video we're gonna talk about ideal gasses and how we can describe what's going on with them. So the first question you might be wondering is, what is an ideal gas?
And it really is a bit of a theoretical construct that helps us describe a lot of what's going on in the gas world, or at least close to what's going on in the gas world. So in an ideal gas, we imagined that the individual particles of the gas don't interact. So particles, particles don't interact. And obviously we know that's not generally true.
There's generally some light intermolecular forces as they get close to each other or as they pass by each other or if they collide into each other. But for the sake of what we're going to study in this video, we'll assume that they don't interact. And we'll also assume that the particles don't take up any volume. Don't take up volume. Now, we know that that isn't exactly true, that individual molecules of course do take up volume. But this is a reasonable assumption, because generally speaking, it might be a very, very infinitesimally small fraction of the total volume of the space that they are bouncing around in.
And so these are the two assumptions we make when we talk about ideal gasses. That's why we're using the word ideal. In future videos we'll talk about non-ideal behavior. But it allows us to make some simplifications that approximate a lot of the world.
So let's think about how we can describe ideal gasses. We can think about the volume of the container that they are in. We could imagine the pressure that they would exert on say the inside of the container. That's how I visualize it. Although, that pressure would be the same at any point inside of the container.
We can think about the temperature. And we wanna do it in absolute scale, so we generally measure temperature in kelvin. And then we could also think about just how much of that gas we have. And we can measure that in terms of number of moles. And so that's what this lowercase n is.
So let's think about how these four things can relate to each other. So let's just always put volume on the left-hand side. How does volume relate to pressure? These mean exactly the same thing. Be careful if you are given pressures in kPa kilopascals. For example, kPa is Pa. You must make that conversion before you use the ideal gas equation. This is the most likely place for you to go wrong when you use this equation.
That's because the SI unit of volume is the cubic metre, m 3 - not cm 3 or dm 3. So if you are inserting values of volume into the equation, you first have to convert them into cubic metres.
Similarly, if you are working out a volume using the equation, remember to covert the answer in cubic metres into dm 3 or cm 3 if you need to - this time by multiplying by a or a million. If you get this wrong, you are going to end up with a silly answer, out by a factor of a thousand or a million. So it is usually fairly obvious if you have done something wrong, and you can check back again. This is easy, of course - it is just a number. You already know that you work it out by dividing the mass in grams by the mass of one mole in grams.
I don't recommend that you remember the ideal gas equation in this form, but you must be confident that you can convert it into this form. A value for R will be given you if you need it, or you can look it up in a data source. The SI value for R is 8. Note: You may come across other values for this with different units. A commonly used one in the past was The units tell you that the volume would be in cubic centimetres and the pressure in atmospheres.
Unfortunately the units in the SI version aren't so obviously helpful. The temperature has to be in kelvin. Don't forget to add if you are given a temperature in degrees Celsius. Calculations using the ideal gas equation are included in my calculations book see the link at the very bottom of the page , and I can't repeat them here. There are, however, a couple of calculations that I haven't done in the book which give a reasonable idea of how the ideal gas equation works.
If you have done simple calculations from equations, you have probably used the molar volume of a gas. You may also have used a value of These figures are actually only true for an ideal gas, and we'll have a look at where they come from. And finally, because we are interested in the volume in cubic decimetres, you have to remember to multiply this by to convert from cubic metres into cubic decimetres.
And, of course, you could redo this calculation to find the volume of 1 mole of an ideal gas at room temperature and pressure - or any other temperature and pressure. The density of ethane is 1. Calculate the relative formula mass of ethane. The volume of 1 dm 3 has to be converted to cubic metres, by dividing by We have a volume of 0. Now put all the numbers into the form of the ideal gas equation which lets you work with masses, and rearrange it to work out the mass of 1 mole.
Now, if you add up the relative formula mass of ethane, C 2 H 6 using accurate values of relative atomic masses, you get an answer of Which is different from our answer - so what's wrong?
The density value I have used may not be correct. I did the sum again using a slightly different value quoted at a different temperature from another source. This time I got an answer of So the density values may not be entirely accurate, but they are both giving much the same sort of answer.
Ethane isn't an ideal gas.
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