OK, so here's the question:

a) Given that *mu* = 0.25 K/atm for Nitrogen, calculate the value of its isothermal Joule-Thomson coefficient.

b) Calculate the energy that must be supplied as heat to maintain constant temperature when 15,0 mol N_{2} flows through a throttle in an isothermal Joule-thomson experiment and the pressure drop is 75atm.

This is what I got so far :

*mu*_{T} = - C_{p} . *mu*

Now, the value of *mu* is given in the Question.

What I would like to know is how do I calculate C_{p} ?

Can I assume that C_{p} = 5/2 R ? Since nitrogen is a monoatomic gas.

So, *mu*_{T} = - ( 5/2 ) (8.314510 J/K.mol) x (0.25K/atm)

= - 5,1966J/atm.mol

But at the back of the book the answer is -7,2 J/atm.mol.

How did they get to this?