 Okay, so a balloon is filled with helium gas and occupies 2.50 liters at 25 degrees Celsius and 1 atm. When released, it rises to an altitude where the temperature is now 20 degrees Celsius and the pressure is only 0.800 atm. Calculate the new volume of the balloon. So is that a problem where things change? Yes. So we're going to have to do what? pv equals nRT over pv equals nRT. So let's write that down and remember we did p1, v1, n1, and R cancel so we don't even have to put that if you don't want to. p2, but I mean if it helps you, let's put R because it seems like some people want to. R's going to cancel already, right, because it's a constant. So do we know any of these things? Do we know, well it says, yeah we know v1, v1 is 2.50 liters and remember the units we want them in. 2.50 liters is a good unit for us. T1, we know, right, that's 25 degrees Celsius plus 273, 298, Kelvin, p1, right, 0, 0, yeah, 1 atm. Did the number of moles change in this problem? Look at the problem. How would the number of moles change if I had a balloon filled with gas? How low would I have to do to it to change the number of moles? Open it and let the gas out or put some more gas in. Did that happen in this? No. So the number of moles is what? The same or different? The same. The same so we can cancel that one, right, do you understand what I'm doing? So do we know these other ones, v2, t2, and p2, yeah, we don't know v2, that's what we're looking for, right, do we know t2, yeah, it's going to be 20 plus 273, 293, Kelvin and p2, do we know that? So essentially our new equation is going to be t1 over t2 equals p1 times p1 over p2 times v2 and we're looking for v2, right, so the first step is to flip both sides of that equation over. So let's just write that over here. So since I want v2 on the left side, I'm just going to do it this way, p2, v2, so we flip that over, right, p1, p1, and that's going to equal what? t2 over t1, right, and we want to isolate the variable v2, can we do it all in one shot? Yeah. How do we do it? Multiply by p1, v1, and divide by p2, right, and then we got to do that same thing to the other side too, right, p1, p1 over p2, like that, so cancel, cancel, cancel, cancel, cancel, and what do we get? v2 equals t2, p1, v1 divided by t1, p2, okay? That's essentially the combined gas law, or this is actually, either one, they all work, and now all we do is plug and check. So do we have all that? t2, p1, v1, t1, p2, yep, just plug them in, so t2, 293 Kelvin, p1, 1.00 ATM, v1, 2.50 liters, plus 2, t1, 298 Kelvin, and p2, 0.800 ATM, and watch, we'll cancel out our units, Kelvin cancels out Kelvin, ATM cancels out ATM, leaving us with liters, is that the units that we would want? Is that the units of volume? So I'm just going to put this up here, so all we've got to do is find our calculator source, and then plug these in through these in, so what do we got here? 293 times 1 times 2.5 divided by 298 divided by 0.8. So if we had a balloon that did all of this stuff, so the thing is, is the temperature decreased, also decreased, so the volume of the balloon is actually going to get bigger, so it's a function of both of those things happening. So the temperature decreasing is going to make your balloon smaller, but the pressure decreasing is going to make it bigger, so the pressure decrease overweighs the temperature decrease, okay? And what I got, hopefully everybody else got the same answer, what did you guys get? 3.07. When we have 298, I was multiplying, and I'm supposed to be dividing. Yeah, you shouldn't be multiplying that, that means dividing. I know that I was multiplying the two at the bottom. Yeah, you should multiply these two by each other. Go watch the video again, okay? I promise you that's what I did, but I've been doing it for a long time, okay? I promise you that's what I did. Are there any more questions? Do it again, I promise you you'll get the right answer. Okay, cool? We cool?