# Equation of State of an Ideal Gas

1. Nov 2, 2008

### LeePhilip01

1. The problem statement, all variables and given/known data
Show that:-
a) the expansivity $$\beta$$ = $$\frac{1}{T}$$
b) the isothermal compressibilty $$\kappa$$ = $$\frac{1}{P}$$

2. Relevant equations
P$$\upsilon$$ = RT where $$\upsilon$$ = molar volume

3. The attempt at a solution
A big mess!

Last edited: Nov 3, 2008
2. Nov 2, 2008

### Mapes

Hi LeePhilip01, welcome to PF. Do you know how the expansivity and isothermal compressibility are defined in general? (Hint: it will involve derivatives.)

3. Nov 3, 2008

### LeePhilip01

Yes, however i wasn't sure whether they were important because they weren't given in th question.

$$\beta$$ = $$\frac{1}{V}$$ . $$\frac{dV}{dT}$$

$$\kappa$$ = - $$\frac{1}{V}$$ . $$\frac{dV}{dP}$$

4. Nov 3, 2008

### Mapes

To be precise, we should say

[tex]\beta=\frac{1}{V}\left(\frac{\partial V}{\partial T}\right)_P[/itex]

[tex]\kappa=-\frac{1}{V}\left(\frac{\partial V}{\partial P}\right)_T[/itex]

to acknowledge that V is a function of multiple variables and that we are taking the partial derivative with respect to one of the variables while holding the others constant.

Now use

[tex]Pv=RT[/itex]

[tex]\beta=\frac{1}{v}\left(\frac{\partial v}{\partial T}\right)_P=\frac{1}{v}\,\frac{\partial }{\partial T}\left(\frac{RT}{P}\right)\right)_P[/itex]

and so on.