## R constant

$PV=nRT$

805097738
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### R constant

When do we know when to use which R constant?

What are the two R constants and when are they used?

Is the unit for gas measured in atm pressure or bars?

Hannah Romeo 1J
Posts: 58
Joined: Thu Jul 11, 2019 12:16 am

### Re: R constant

R is used in the ideal gas law expression, PV=nRT. A specific R constant is used as a result of the given units in the problem. For example, if pressure has units of atm, R has a value of .0821 (L x atm)/(mol x K)

Jordan Ziegler 2J
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Joined: Sat Aug 17, 2019 12:15 am

### Re: R constant

Units of gas can be expressed in atmospheres or in bars. However, the textbook uses mostly bars to express partial pressure. I would be prepared to do some conversions between the two units though.

Qiu Ya Wu 4I
Posts: 107
Joined: Fri Sep 20, 2019 12:17 am

### Re: R constant

Lavelle gives you 4 different values for the R constant on his formula sheet during tests. Whichever one you use depends on the units given in the problem.

Areli C 1L
Posts: 95
Joined: Wed Nov 14, 2018 12:19 am

### Re: R constant

Qiu Ya Wu 4I wrote:Lavelle gives you 4 different values for the R constant on his formula sheet during tests. Whichever one you use depends on the units given in the problem.

Do you know if the problems will mention directly which R constant to use?(Like a given)

Sanjana K - 2F
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### Re: R constant

It generally won't tell you exactly what R value to use, but all the constants are given in the formula sheet so depending on whether pressure is reported in atm or kilopascals, you'll know which R-value to use.

Areli C 1L wrote:
Qiu Ya Wu 4I wrote:Lavelle gives you 4 different values for the R constant on his formula sheet during tests. Whichever one you use depends on the units given in the problem.

Do you know if the problems will mention directly which R constant to use?(Like a given)

Sean Cheah 1E
Posts: 105
Joined: Wed Sep 18, 2019 12:20 am

### Re: R constant

As previous posters have already noted, pay careful attention to the units of the given values in each problem when determining which R constant to use. For example, when converting from Keq to delta G, the correct R value would be about 8.314 $J*K^{-1}*mol^{-1}$, which makes sense seeing as your final result should be expressed in Joules.