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Leslie Almaraz 4G wrote:How do you know which variation of the R constant to use?
I know that Test 1 is already over, but in the equations sheet that Dr. Lavelle provided, there were many variations of it. To answer your question, the best way to know which R constant to use is to see the units used in the problem. If it mentions atm, then it'll be the number with atm in the units. Same applies for J, and so forth.
Shail Avasthi 3C wrote:The value of R you use depends on the units given to you in the problem and the units which your answer needs to be in. The units of R will cancel out with all of the given terms to give you your final answer's units.
The R constant is dependent on the units of pressure you are given. Looking up a table is helpful and just pay attention to the units you are given in the question. You don't need to memorize this, just know which values correspond to each unit of pressure.
To determine which R to use view the givens you have and then you can determine which R unit would cancel out the appropriate units to get what value you desire. For most calculations, like in the VP=nRT equation, it will be 0.08206.
There are a lot of gas constants, so just fit the units that are involved with the units of the correct gas constant. Generally, I use 0.0821 Latm/(molK) when I'm trying to find volume or pressure and 8.314 J/(molK) when I'm trying to find energy.
When looking at the units, it's good to note which ones will cancel out, and to keep in mind the units that your answer is supposed to be in. For the ideal gas law, you will typically be using any value with a unit of pressure (atm, bar, Torr). When calculating anything related to energy, the values with units of J or L/atm (1 L atm = 101.325 J) are helpful.
It will normally depend on the units given for the other quantities being used in the problem, and the variations of R and its value/units will usually be given on the formula sheet, so it is easier to assess the units on a case by case basis and see which one is the best fit.
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