Here the example question asks, how many moles of chlorine gas occupy a volume of 15.7 liters at STP? Alright. So here they're talking about determining the moles of an ideal gas. They're giving us the volume of that gas at STP. Here we can use the conversion factor that we know exists with the standard molar volume. We're going to say we have 15.7 liters, and we're going to say here that the conversion factor is for every 1 mole of any gas at STP, the volume is 22.4 liters. Here, liters cancel out and I'll have my moles, which comes out to 15.722.4 moles of Cl2. So this is one way that we get our answer.
What else we could do is we could have also said that we have 15.7 liters which is our volume, and then we have STP which is pressure and temperature. We could have said that our moles equals PV/RT and we would have gotten the same exact answer because here this would have been 1 atmosphere. This here is 15.7 liters. Then here we have our R constant with its units, don't forget the units, times temperature at STP is 273.15 Kelvin. And if we worked it out we get the same exact moles for Cl2. So just realize that there are two ways that we can approach a question like this. Using it with the conversion factor of 1 mole for every 22.4 liters, or by using it through the traditional means with the ideal gas law.