Learn about Junior Chemistry, Gases 2, in this comprehensive video by bannanaiscool.
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Rob Lederer: About 150 years ago, there was an experimenter named Boyle and he came up with his law for gases that sounds pretty intuitive to the rest of us. If you take a gas in a certain volume and pressurize and increase the pressure what you have done is that you've decreased the volume. So, what did Boyle do? He took depression of a certain amount of volume of gas and he kept the moles constant of that gas in the sample and he changed the volume, and then recorded the pressure. Whenever he took the pressure and the volume and multiplied them together, he got a number, but if he changed it to a different pressure and responded with a different volume or vice versa when multiplying the pressure and the volume together you always got that number again. Well, of course, it's a constant K. And so, he said you know that also means that, if you have an initial pressure and volume of a gas and you want to know what its final volume is going to be after you change the pressure or you could use this formula. And ladies and gentlemen that's Boyle's formula right there. P1V1=P2V2, very, very convenient for being able to find an unknown in pressure or volume. When you keep the temperature constant, that's Boyle's law, at constant temperature the pressure and volume of a gas vary, they vary with each other indirectly. Now, what does that mean? Look, that means as one goes up, the other goes down, right? So, if you are going to graph Boyle's law, pressure versus volume what do you do? Well, as one goes up the other goes down, so a lot of people think Oh! Why don't we just do something like this, a straight line in this way? So, when the pressure is going down, the volume is going up. A nice idea, except for one thing, if you cross that wire x-axis here, what you are going to do is encounter a zero for pressure or volume. If you get a zero for pressure or volume, you do not get the constant K anymore. So what does that graph actually look like? It's a curve where you have asymptotes really along that Y and X-axis here and that's the graph for this. If you actually graphed real quick, if you wanted to make a straight line, you can make the relationship this. One over the volume and you can check this anytime you want to, if you graph one over the volume and pressure with each other then you will get that straight line relationship that you've been looking for and it's actually a positive slope. Here is a question at employs Boyle's law. You got 200 milliliters of gas and the pressure inside of that container, closed container that has 200 milliliters in it is 100 KPA and now we take the volume and we go to 100 milliliters. So, what if we do? We've taken 200 milliliter container and squeeze it down into 100 milliliters, what's the pressure now? The pressure has gone up, hasn't? How much has it gone up? You use Boyle's formula to find out. I think we probably know what the answer is already. So, P1V1=P2V2; P2 is what we are looking for -- the new pressure what is it equal? P1 times V1 divided by V2, you divide each side by V2 write the new formula; then plug in the numbers. So, P2 equals this over here that's the way you do it properly. Now, 200 is the original volume, I would actually kind of reverse those, haven't I? It's okay. So, there is a volume, there is your first pressure, there is your new volume, and of course, when you do the math you get 200 KPA, the pressure goes up, the pressure doubles well. Well, if you take that volume and you half it, the pressure is going to double as long as the temperature remains constant.