How to Find the Number of Molecules From Volume

So long as you know volume you can find the number of molecules in just a few quick moments. Find the number of molecules from volume with help from a teacher with over 20 years of experience in this free video clip.

Part of the Video Series: Chemistry and Physics Calculations

Video Transcript

Hi I'm Janice. I teach science here in Clearwater, Florida and today I'm here to show you how to convert from volume to molecules in chemistry. So let's say for instance that we have one liter which is volume, a liter is a measure of volume of carbon dioxide and we wanted to know how many molecules that was. Well in chemistry we can't actually convert direction from liters to molecules. We have to go through the mole. The mole is going to be our consistent unit of measure and the mole is also going to be necessary if we want to convert from one substance to another. So there's a couple of conversation factors that we need to use. The first one is this, one mole of any gas equals 22.4 liters. So that means a mole of carbon dioxide is going to have a volume of 22.4 liters of carbon dioxide. Now one mole of any substance also has a fixed number of molecules, 6.022 times 10 to the 23rd or what we call Avogadro's Number. So these are the conversions we're going to have to use to go from liters to moles to molecules and we're going to set up something called a dimensional analysis or conversion problem that's going to look like this. So I would start with my one liter of carbon dioxide because I always start with what I'm given. I'm going to draw a nice long line like that and one of the rules in these conversion problems, dimensional analysis, is that if I have liters of carbon dioxide on the top here, on the other side of this bar I'm going to have to have it on the bottom. So, I said that there's 22.4 liters in one mole. So down here I'm going to write 22.4 liters of carbon dioxide is one mole of carbon dioxide. Now if I was converting from one substance to another I would put a mole to mole ratio in here. But for right now we're just going to stick with the same substance. So, I'm now in moles of carbon dioxide. I'm going to check that by crossing these off. They're the same so I can cancel them out but now I want to go from moles to molecules, alright and I'm going to use this number right here to do that. So I have moles up here. So I'm going to put moles on the bottom down there and 6.022 times 10 to the 23rd molecules, cross out my units. This is all I'm left with and now I just have to do the math, 1 times 1 times 6.022 times 10 to the 23rd divided by 22.4 times 1. So really when I do the math that's just going to be 6.022 times 10 to the 23rd divided by 22.4 and that's going to be the same for any gas that you're dealing with because these conversion factors stay the same. So that's how you convert from volume to molecules and to go the other way, you just go backwards. I'm Janice. Have a great day.

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