R constants
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R constants
How do we know which value of R to use because aren't there two values? I know we usually use 8.3145, but when would we use the other value?
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Re: R constants
It depends on the constants and what needs to cancel. Both values should be provided so you just have to look at the question. It should usually be 8.314 J/k.mol
Re: R constants
Whenever using the ideal gas equation (pv=rnt) we want to use the R constant with units of L*atm*mol^-1*K^-1 because the units will cancel.
When solving for work or entropy, we usually use the one that 8.314 J/mol*K because the units cancel out and leave us with J.
In order to be absolutely sure, just take a look at the units you already have, consider the units you want for your answer, and select the R-value with units that will get you to your desired answer.
When solving for work or entropy, we usually use the one that 8.314 J/mol*K because the units cancel out and leave us with J.
In order to be absolutely sure, just take a look at the units you already have, consider the units you want for your answer, and select the R-value with units that will get you to your desired answer.
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Re: R constants
One of the good things about the final is we are given the constants sheet with all the gas constants given to us, but like previously stated, the best way to approach a problem where you have to use an R constant is to look at the values and units given and use the constant with most (if not all) the units and you should be able to come to the right answer without error
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Re: R constants
The value of R changes based on the units. All of them are given on the equation sheet, so just look to see what units the problem has given you and choose the appropriate one
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Re: R constants
It is important to look at the units of what the problem includes. All of the R constants will be provided on the equation sheet, so when you need to use the R constant, consider what units you are using and use the constant that aligns with these units.
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Re: R constants
A quick way of determining which R value to use is to look at the units given to you. The units you would need to cancel out should correspond with that of the R value you use.
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Re: R constants
it depends on what units are given in the problem; each R value has a different set of units
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Re: R constants
For which problems on the equation sheet will we ever use R=8.314*10-2 L*bar*K-1*mol-1?
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Re: R constants
It generally depends on what units are given to you in the problem, then you base which R to use off of that.
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Re: R constants
So we would look at what would cancel out in order to get the answer we want for example L, atm, J
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Re: R constants
I always look at the units of the information given to me and find the value of R that has most of the same units.
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Re: R constants
Look at the units of the constant and units of the problem to determine which constant to use! Usually, when doing thermodynamics we will be using 8.314. Hope that helps!
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Re: R constants
It primarily depends on the units used; as a general rule 0.08206 works when you're using ideal gas law and 8.314 works for calculating deltaS since it does the conversion from L atm to joules for you
Re: R constants
it depends on the units of the other variables in the equation/calculation you are doing.
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Re: R constants
You just have to make sure that you have the right units with the R value. For the most part in thermochemistry we used 8.314 but like in an instance of using PV=nRT, we would use the other R values.
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Re: R constants
Make sure your units cancel out and the unit for pressure (bar, atm, Torr) matches the unit for the pressure value in the question.
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Re: R constants
You would use the constant that has all the units that you need to cancel out
so if you are given a volume in L, a pressure in atm, and a temperature given in K, you would use R=.0206 L*atm/(K*mol)
so if you are given a volume in L, a pressure in atm, and a temperature given in K, you would use R=.0206 L*atm/(K*mol)
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Re: R constants
As everyone said, I would look at the units you are given and the units you are trying to reach and choose the one the fits where you can cancel out the units u don't want in the final answer
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Re: R constants
You just have to look at the units and pick the value of R that will cancel them out.
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Re: R constants
depending on the unites, we will use a different value of R. all are listed on the constants sheet. if you have a problem that has L.atm, you would use .08206. if the problem is with joules, you use 8.314
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Re: R constants
Hi! It depends on the units that you're trying to cancel. When using the ideal gas law, it is usually .08206L*atm(or bar)/K*mol. When trying to solve for J you would use 8.314 J*/K*mol. Hope that helps! :)
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Re: R constants
Try to remember the units of R in order to solve. Look at the context of the situation by looking at the units in the equation you're trying to solve, and make sure that the R you use properly cancels out the units.
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Re: R constants
The equation sheets have the units so make sure to check the units of given information.
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Re: R constants
We use different R's depending on the units of whatever you're trying to calculate. It helps to write out a complete equation with units since you can see what cancels and which R is needed to complete the equation.
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Re: R constants
For this class, most of the time we use the 8.314 constant because we need it to cancel units. Everything depends on the units you are working with.
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Re: R constants
Make sure to look out for what units you are given. Those can help you determine what R constant would be the best to use.
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Re: R constants
I always look at the units and go from there. When there are liters and atm, the 8.206 x 10^-2 is teh correct R.
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Re: R constants
You'll use the other R values when you have different units. For example you use 8.314 when your units include Joules. 8.206x10^-2 on the other hand is L(atm)/(K)(mol). This is typically used for things like the ideal gas equation, but its best to check your units each time.
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Re: R constants
You would most likely have to look at the units given and then from there determine which R value/constant is best to use.
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Re: R constants
It depends on the problem. You have to look at the constants but the majority of problems use the constant 8.314.
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Re: R constants
Usually I like to look to see which units are given for other values in the problem and go off of that :)
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