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The right gas constant R depends on the units for pressure and volume that the question uses. For example, if the question has the pressures measured in atm and the volume measured in liters, you would need to use 0.082057 L*atm*mol^-1*K^-1 for R.
The most common R values are 8.3145 J/(K*mol) and 0.08206 (L*atm)/(K*mol). Choosing which one to use depends on the given units of the other variables in the equation. For example, if the pressure (P) is given in units of bar pressure, you can convert bar pressure to atm using dimensional analysis and use R = 0.08206 (L*atm)/(K*mol) to solve. Another example would be if pressure is given in Pa, then you could convert and use R = 8.3145 J/(K*mol) to solve. Hope this helps :)
It depends on the units that are given in the question. One R value is in J/K*mol and the other is L*atm/K*mol, so you would look at what units are present or that you could convert to and use the corresponding R value.
You would choose the value of R based on the units you choose to use for that particular problem. For example, if you look the the R value with units J/K*mol, then you are trying to cancel out a value with units "J" and if you would use the R value with the units L*atm/K*mol, when you are trying to cancel out a value with units L*atm.
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