Gas Constant Value
Moderators: Chem_Mod, Chem_Admin
-
- Posts: 104
- Joined: Wed Sep 30, 2020 9:49 pm
- Been upvoted: 1 time
Gas Constant Value
Hello!
I was wondering which gas constant we're supposed to use in this class when doing calculations with the ideal gas equation. I know there's multiple values for the gas constant, but the one I've seen most used in this class seems to be .08205. So is this gas constant the one we'll predominately use, or should I memorize all gas constants in case we have to use a different value (depending on units)?
I was wondering which gas constant we're supposed to use in this class when doing calculations with the ideal gas equation. I know there's multiple values for the gas constant, but the one I've seen most used in this class seems to be .08205. So is this gas constant the one we'll predominately use, or should I memorize all gas constants in case we have to use a different value (depending on units)?
-
- Posts: 102
- Joined: Wed Sep 30, 2020 10:00 pm
Re: Gas Constant Value
Hi there! Which gas constant value you use typically depends on what the units you're working with are. For most of the problems I've seen that would be the one we're using the most so I think it would be fine to stick with that one, but it all depends. Hope this helps, and please correct me if I'm wrong!
-
- Posts: 100
- Joined: Wed Sep 30, 2020 9:51 pm
Re: Gas Constant Value
Hello! On the "Constant and Equations" sheet (found on Professor Lavelle's website), the sheet has all the different gas constants for different units that may arise. I believe we are permitted to use the Constants and Equations reference sheet on the midterms, so you can simply refer to the sheet if you need to know a gas constant for a question.
Hope this helps!
Hope this helps!
-
- Posts: 134
- Joined: Wed Sep 30, 2020 10:11 pm
Re: Gas Constant Value
Hi! the gas constants are on the constants and equations sheet found on Lavelle's website, and we can use that sheet during tests. therefore, you don't necessarily have to memorize all of them if they are on the sheet, but I guess use the one that has the correct units for the problem. Hope this helps!
-
- Posts: 100
- Joined: Wed Sep 30, 2020 9:53 pm
- Been upvoted: 1 time
Re: Gas Constant Value
Hi! The value we use for R generally depends on the units! .08206 (L atm)/(mol K) and 8.314 J/(mol K) are probably the ones we'll use most tho. Just make sure that the units on the right and left are the same, and you'll be good!
-
- Posts: 62
- Joined: Tue Nov 17, 2020 12:20 am
Re: Gas Constant Value
During exams, we can use an equation sheet (which is on professor's website) that contains I believe three R-values... Those are the only R-values that we are given to use on exams, from what I've heard. However, those three given R-values are all we need to do most problems, so I would recommend practicing using those values!
-
- Posts: 108
- Joined: Wed Sep 30, 2020 9:59 pm
Re: Gas Constant Value
https://lavelle.chem.ucla.edu/wp-conten ... ations.pdf. This has the gas constant value with different units!
-
- Posts: 60
- Joined: Wed Nov 18, 2020 12:18 am
Re: Gas Constant Value
Hey! To answer your questions, you have to look at the units of the other components of the problem in order to make a decision on what gas constant you want to use. Most of the time, pressure is in atm so we want to use the gas constant 0.0821 (L atm)/(K mol). If our pressure was in kPa or mmHg, we would use the gas constants that correspond to those units.
-
- Posts: 103
- Joined: Wed Sep 30, 2020 9:50 pm
Re: Gas Constant Value
To decide which one to use you would need to look at the units for the values that you calculated/given and use the constant that matches those units.
-
- Posts: 80
- Joined: Wed Sep 30, 2020 9:44 pm
Re: Gas Constant Value
Hi! With the gas constant vary, and you just need to make sure that the units line up!
-
- Posts: 104
- Joined: Wed Sep 30, 2020 9:33 pm
- Been upvoted: 1 time
Re: Gas Constant Value
The gas constant value will be on the equations sheet that we are allowed to use on the midterm and final. Although there are different R values with different units, you use the one that has the correct units necessary to cancel or that goes with the problem.
-
- Posts: 100
- Joined: Wed Sep 30, 2020 9:33 pm
Re: Gas Constant Value
Hello, for this class we should be using the gas constant given in the equations/formula sheet Lavelle provided us for exams. We don't need to memorize it!
-
- Posts: 105
- Joined: Wed Sep 30, 2020 9:50 pm
Re: Gas Constant Value
You would have the check the pressure units to figure out what constant to use and you don't need to memorize them because they can all be found on the Constants & Equations Sheet on Lavelle's website.
-
- Posts: 100
- Joined: Wed Sep 30, 2020 10:00 pm
-
- Posts: 52
- Joined: Wed Feb 19, 2020 12:19 am
Re: Gas Constant Value
Does anyone know if Lavelle is also strict on significant figures? I just want to make sure that I do not loose easy points on the midterm, so if anyone who has taken him previously could say, I would appreciate it!
-
- Posts: 62
- Joined: Tue Nov 17, 2020 12:20 am
Re: Gas Constant Value
Alexis Sanft 1E wrote:Does anyone know if Lavelle is also strict on significant figures? I just want to make sure that I do not loose easy points on the midterm, so if anyone who has taken him previously could say, I would appreciate it!
I didn't have him previously, but I asked a couple TAs and they told me not to worry about it! So you should be fine!
-
- Posts: 55
- Joined: Wed Nov 18, 2020 12:29 am
Re: Gas Constant Value
Alexis Sanft 1E wrote:Does anyone know if Lavelle is also strict on significant figures? I just want to make sure that I do not loose easy points on the midterm, so if anyone who has taken him previously could say, I would appreciate it!
Also, the midterms are multiple choice so usually you should be able to select the answer you got even if it's a few significant figures off! But just in case, I would generally use exact values until you get to the final answer to avoid possible rounding errors
Re: Gas Constant Value
The value is on the formula chart, and like what others have said make sure the units are correct!
-
- Posts: 108
- Joined: Wed Sep 30, 2020 9:51 pm
Re: Gas Constant Value
The two gas constant values (that I know of) are 8.314 J/(mol x K) and 0.08206 (L x atm)/(mol x K) and I think it just depends on what units the problem is asking the answer to be in, along with what units the problem gives to you. Both are on the equation sheet we're allowed to use during the midterms and final.
-
- Posts: 113
- Joined: Wed Sep 30, 2020 9:55 pm
Re: Gas Constant Value
When determining which gas constant value to use we have to consider the units given in the problem. We have (R=8.314 J*K^-1mol^-1) and (R=0.08206 L*atm*K^-1) as our two gas constant values, and matching units to make sure they cancel in the problem is essential for receiving the desired output.
-
- Posts: 121
- Joined: Wed Sep 30, 2020 9:35 pm
Re: Gas Constant Value
You must look at the units you are working with to determine this because each gas constant has different units.
-
- Posts: 111
- Joined: Wed Sep 30, 2020 9:51 pm
-
- Posts: 118
- Joined: Wed Sep 30, 2020 9:52 pm
- Been upvoted: 1 time
Re: Gas Constant Value
Since there are several different values for the gas constant that differ, depending on the units, look at the units you are dealing with in the problem and choose the appropriate R value (that is, make sure that units match up). The best way to double check whether you chose the correct value for R is to make sure all units cancel out when you do your calculation.
-
- Posts: 100
- Joined: Wed Sep 30, 2020 9:40 pm
- Been upvoted: 2 times
Re: Gas Constant Value
All the gas constants are on the equation sheet, and you will be able to figure out which one is the proper one to use by looking at the units and seeing if it corresponds with the rest of the units you are using in your problem.
-
- Posts: 100
- Joined: Wed Sep 30, 2020 9:52 pm
Re: Gas Constant Value
It all depends on the problems. Some discuss certain units so you would use the constant that include those said units.
-
- Posts: 125
- Joined: Wed Sep 30, 2020 9:31 pm
- Been upvoted: 3 times
Re: Gas Constant Value
It depends on what units are given on the problem and you can work based off of that.
-
- Posts: 115
- Joined: Wed Sep 30, 2020 10:03 pm
- Been upvoted: 1 time
Re: Gas Constant Value
Hi! It all depends on what is given in the problem. Usually the first thing I look at is the units of pressure. If there is no pressure given, I will go with the R value that has no pressure in the units!
-
- Posts: 118
- Joined: Wed Sep 30, 2020 9:55 pm
Re: Gas Constant Value
The gas constant you are suppose to use always depends on your units. Make sure to write down your units when doing a problem to make sure not to mix them up. The way I do it is I look to see if the values are in L.atm or joules. This helps a lot. Make sure things cancel out in the end or you will get the wrong value.
-
- Posts: 115
- Joined: Wed Sep 30, 2020 10:08 pm
- Been upvoted: 1 time
Re: Gas Constant Value
What examples are there for using R=8.314 J*K^-1mol^-1) because I'm still kind of confused why it seems to be used interchangeable with the other R value
-
- Posts: 122
- Joined: Wed Sep 30, 2020 9:58 pm
Re: Gas Constant Value
Thomas Vu 1A wrote:What examples are there for using R=8.314 J*K^-1mol^-1) because I'm still kind of confused why it seems to be used interchangeable with the other R value
R = R=8.314 J*K^-1mol^-1 is used, for example, when trying to find the work done by a isothermal, reversible system (w = -nRT ln (Vf/Vi). The different R values are not interchangeable because the units are different. A shortcut to determining the appropriate R value is to look at the units of the other components of the equations. Units, just like numbers, can cancel each other out.
-
- Posts: 101
- Joined: Wed Sep 30, 2020 9:43 pm
Re: Gas Constant Value
Thomas Vu 1A wrote:What examples are there for using R=8.314 J*K^-1mol^-1) because I'm still kind of confused why it seems to be used interchangeable with the other R value
A good way to understand the difference between the two is that the one with the one with the L atm unit can be multiplied by the conversion of 1 Latm=101.325 J on the equation sheet to equal the one with the Joules unit. So, just check which units align with the corresponding R value units.
-
- Posts: 106
- Joined: Wed Sep 30, 2020 9:54 pm
Re: Gas Constant Value
The gas constant values are .08206 (L atm)/(mol K) and 8.314 J/(mol K). They both count as R, but make sure to remember the difference in using both of them. The first R value is used for problems dealing with ideal gas constant (so the equation PV = nRT). On the other hand, the second value is used for energy (note that the units cancel out and joules is left).
Re: Gas Constant Value
Yes, as everyone mentioned above, you must first check the units! That's the most important thing. But now that we're doing a lot of thermodynamics and work problems (which has the unit of Joules), we'll be using R= 8.314 J/Kmol a lot more often.
-
- Posts: 106
- Joined: Wed Sep 30, 2020 9:59 pm
- Been upvoted: 2 times
Re: Gas Constant Value
I think there are 4 values that are used more often:
R = 0.0821 L·atm/(mol·K) = 8.3145 J/(mol·K) = 8.2057 m3·atm/(mol·K) = 62.3637 L·Torr/(mol·K)
R = 0.0821 L·atm/(mol·K) = 8.3145 J/(mol·K) = 8.2057 m3·atm/(mol·K) = 62.3637 L·Torr/(mol·K)
-
- Posts: 104
- Joined: Wed Sep 30, 2020 9:55 pm
Re: Gas Constant Value
I don't think you need to stress that much over it (at least for tests) since it's on the formula sheet Lavelle provides.
-
- Posts: 51
- Joined: Wed Nov 18, 2020 12:30 am
Re: Gas Constant Value
Hi, a lot of people are saying just to look at the units and use the one that matches or cancels out nicely, and I guess I'd just like to comment on how this "follow the units" idea can be really helpful for many other chemistry problems. Sometimes, if you read a question and have no idea how to answer it, you can just see all the given values from the problem and their corresponding units, then look for a formula or two that allows you to somehow cancel out all the units until you are left with one corresponding to the answer you're supposed to find. Of course, this won't be applicable to every question out there, but it's a good place to start if you're completely stumped by a question!
-
- Posts: 108
- Joined: Wed Sep 30, 2020 9:40 pm
Re: Gas Constant Value
They're all on the sheet we get for exams so no need to memorize, however you should be familiar with how to use them depending on units.
-
- Posts: 111
- Joined: Wed Sep 30, 2020 9:36 pm
Re: Gas Constant Value
The units usually depend on the problem being solved, and the other units given in the question. The units are usually given on the exam sheet, so we just need to know how to apply everything.
-
- Posts: 118
- Joined: Wed Sep 30, 2020 10:04 pm
- Been upvoted: 2 times
Re: Gas Constant Value
You decide which value to use by looking at the units. You choose which is the most desirable to get the units of the value you want.
-
- Posts: 100
- Joined: Wed Sep 30, 2020 9:48 pm
Re: Gas Constant Value
The different gas constants are based on the units that make them up. As a result, determine the units that are given/desired and pick your gas constant based on that value.
-
- Posts: 100
- Joined: Wed Sep 30, 2020 9:34 pm
-
- Posts: 103
- Joined: Fri Aug 30, 2019 12:18 am
Re: Gas Constant Value
there are different values, but use the one that corresponds with the units given
Who is online
Users browsing this forum: No registered users and 6 guests