[Bioionic]

I am somewhat confused about something.  Thus far, when I work gas problems I have been using PV=nRT, and value for R has been .08206
Now I have a lab where we are doing Enthalpy of vaporization.
In the lab equations, we have to compute number of moles at 0 deg C.
The given eqn is  N_air= (P_air)(V_air)/RT.
So I was thinking I would use R = .08206, but in the lab book it says R= 8.314.
So now I am confused.  Which R do I use?  If the latter, what value do I use for V, would it be mL or L.
Or is R = 8.314 just used for the enthalpy eqn?
I am stuck, any help would be great.
Thanks

[Rabn]

Take a nice hard look at the units of each constant.  They are equivalent, but the units are different.  The lab book gave you the thermodynamic R value so you would end up with units of KJ.  You really need to begin considering units in a very serious way.  Without proper dimensional analysis skills you will find yourself in a lot of trouble.

[Bioionic]

From what I could find, R=8.314 is used when using Kpa.  So I would then think we would never use this constant since we always work in atm, in fact our instructor has never even discussed Kpa.  So I still not see how this works if I am always working with atm.  But when doing the Clausius-Clapeyron Eqn, R = 8.314.

I do not get this at all.

R = 8.314 J K-1 mol-1 if Pressure is in kilopascals(kPa), Volume is in litres(L), Temperature is in Kelvin(K)
R = 0.0821 L atm K-1 mol-1 if Pressure is in atmospheres(atm), Volume is in litres(L), Temperature is in Kelvin(K)

[Yggdrasil]

You can convert between joules and L atm in the following manner:

First start with these basic unit conversions:
1 atm = 101.3 kPa
1 kPa = 1,000 N m^-2 =  1,000 kg m^-1 s^-2
1 L = 1 dm³ =  0.001 m³
1 J = 1 N m = 1 kg m² s^-2

Therefore 1 L atm = 0.001 m³ * 101.3 kPa = 0.1013 kPa m³ = 101.3 N m^-2 * m³ = 101.3 N m = 101.3 J.

With the conversion, 101.3 J = 1 L* atm, you should see that the two values of R are equivalent.  Also, if you find the value of R when expressed in units of L kPa mol^-1 K^-1, you should find why 8.314 works when your pressure is in units of kPa.

Being comfortable interchanging between different sets of units is essential to doing any sort of science (remember, the Mars Climate Orbiter crashed on Mars instead of orbiting it because the scientists in charge didn't use the proper units in their calculations).  It is definitely worth reading over again/asking your teacher about.

[edit: correcting spelling mistake]

[Bioionic]

Thank you.  I will try to see if I can work these numbers out. 

[Borek]

Additionally, see wikipedia for a set of R values expressed in different units. Try to interconvert them for training.