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Units

Posted: Sun Jan 13, 2019 6:19 pm
by Dustin Shin 2I
There have been several different units that have been floating throughout my discussion and lectures so far. What is torr? What is bar? How do we find these and are these related in anyway?

Re: Units

Posted: Sun Jan 13, 2019 6:21 pm
by mcredi
The torr is a unit of pressure and is defined as exactly of a standard atmosphere. A bar is a pressure unit defined as 100 kilopascals. This makes one atmosphere nearly equal to one bar. 1 bar = 750.06375541921 Torr

Re: Units

Posted: Sun Jan 13, 2019 6:34 pm
by Chem_Mod
There are several difference units that can be used to describe the pressure of a gas (bar, atm, mmHg, torr etc...). While any of them can be used to describe the pressure of a gas, when doing calculations using the ideal gas law (PV=nRT), you need to make sure to use the correct gas constant (R) which is determined by what unit of pressure you want or by what unit of pressure you are given. In general, the books usually uses atm (which has a R value of ~0.082).

Re: Units

Posted: Sun Jan 13, 2019 6:38 pm
by Eva Guillory 2E
According to google...

torr = "a unit of pressure used in measuring partial vacuums, equal to 133.32 pascals"
bar = "a unit of pressure equivalent to 100,000 newtons per square meter or approximately one atmosphere"
atm = "a unit of pressure defined as 101,325 Pa"
pascal = "the SI derived unit of pressure used to quantify internal pressure"; "one newton per square metre (kg * m^-1 * s^-2)"

This site gives a pretty good idea of what pressures to use for various conditions like STP, SATP, etc.:
https://www.engineeringtoolbox.com/stp- ... d_772.html

If the pressure is not defined, you can use a modified version of the ideal gas law (P = (n/V)RT, where n/v = concentration) to find it if you are given the other units.