so there are many pressure units uh there are probably like two maybe three that we tend to use in chemistry um but there are more than that so we're just going to go through them today and talk about their conversions a little bit so millimeters of mercury is sort of the first one that was developed it's the first way we sort of measured atmospheric pressure a barometer was used which you can kind of see here um i think this is technically a manometer um but uh basically you have some gas in here with a uh like known pressure or you could have a known pressure on the outside i mean really the known pressure could be either one of the two and then you measure like the millimeters of distance that the mercury travels to compare and so that's why the the symbol is millimeters of mercury it's because you're you're actually measuring the the distance of like the top of the mercury to the bottom of the mercury or i guess because it's like this u shape so it's like top to top but really it's that difference there that you're measuring so you can see that the mercury sort of goes up to here and here so you measure that distance to be able to compare the pressure of the two gases uh the guy that made that up his last name was like toricelli i want to say um something like that so sometimes we call it tor but millimeters of mercury was sort of our first way of measuring atmosphere so from there we there's been a lot more that have been developed um atmospheres or atm is the symbol that's one that we very very commonly use it's sort of with the the standard like sea level zero degrees celsius is one atm so it's just sort of assigned that one atm would be would coordinate with that like level of the atmosphere and the temperature standard temperature and pressure so stp that's a term you're going to hear a lot and it's it's just a a way at which we measure gases standard temperature and pressure it means zero degrees celsius and one atm when we do calculations with temperature in this unit we typically convert to kelvin so in order to get from celsius to kelvin you add 273 so 0 degrees celsius is equal to 273 degrees 273 kelvin sorry kelvin is not degrees celsius is though kelvin is more of a scale it's a measurement of energy we call it the absolute temperature you can't have a negative kelvin i mean you can't have a zero uh you can't have a negative kelvin zero kelvin is the coldest temperature i guess you could you could say but like i said kelvin's not really a measurement of temperature it's more of a measurement of energy so basically with no energy if an atom had absolutely no movement it would be at 0 to 0 kelvin but that is sort of a theoretical number even the most the coldest molecules still have a tiny bit of movement the conversion between atm to millimeters of mercury and torque is remember torn millimeters of mercury are one to one um but atm if atm is one that means uh millimeters of mercury is 760. so that's the conversion factor that you're going to use to convert and i'll show you examples of those in your notebook um as far as your like mole goes when you're converting to grams use grams to mole like from the periodic table the molar mass when you're converting to molecules you use the avogadro's number 6.022 times 10 to the 23rd if you're converting to volume at stp the constant that you use is 22.4 so that is a number that you should know pascal is another way that we measure pressure uh the symbol is pa however we typically use kpa kilopascals as our um as our unit uh just because a pascal is is very very big when you compare it to atm um uh one pascal is is equal to one newton per meter squared so it was actually created by a mathematician not a scientist um we do use it it's we use it less often than we use atm or millimeters of mercury but it is one that we do use on and off the conversion is 101.325 kpa equals 1 atm which again equals 760 millimeters of mercury which also equals 760 torr so all of these numbers are equal to each other they can all be used as conversion factors again we use kilo pascals because pascals would be 101 325 that's just a really big number so 101.325 is just easier to work with so we use kilopascals typically not pascals american units here in the united states we use psi which stands for pounds per square inch pounds and inches are not something that we use in science therefore we also don't use psi in science however if you are a diver um usually the um the the gas and your the air in your gas in your like tank that you wear on your back that's measured in psi so when you see your um when you when you look at your they're called like secondaries and stuff like that so i don't i can't remember what the actual name is but when you um look at it it's measured in psi it's on a little it's in like a little um dial uh we also do psi and tires so if you like look closely at your tires um usually there will be a psi label there this one also actually also has a kpa label so that's pretty neat um but psi is something you'll typically see on your tires too um uh one atm which is equal to 760 torr which is equal to 760 millimeters mercury which is equal to 101.325 kpa is also equal to 14.7 pounds per square inch a lot of times when you google that you get 15. we're going to use 14.7 a little more specific in here again so the value for psi is 14.7 that is equal to one atm for conversion factors uh in your notebook i will show you examples about how to use these as conversion factors so stay tuned for that