# Homework Help: Calculate the number of moles in a real gas

1. Jan 19, 2010

### larsb

1. The problem statement, all variables and given/known data
I would like to know how to calculate how much moles of gas I have in the following in a cylinder with a certain volume and pressure.

The gas in the cylinder is a mixture of air, with added oxygen and helium, the mixture is 18% Oxygen, 36,6% Nitrogen, 45% Helium and 0,4% Argon. The cylinder is 24 liters big, the pressure in the cylinder is 200bar.
I can calculate the a and b values for the gas mixture.

P = 200 bar
V = 24 L
T = 293,15 K
R = 0,083145 L bar K-1 mol -1
a = 0,8746 L2bar mol-2
b = 0,0388 L mol-1

2. Relevant equations
PV = nRT ( ideal gas law)

P = nRT (V-nb)-1 - n2a V-2

3. The attempt at a solution
According to the ideal gas law this should be 196,93 moles, but that is not right, since if I use the Vanderwaals equation I end up at a pressure of 234,6 bar to accomodate 196,93 moles of this gasmixture.

Using the Vanderwaals equation I can't calculate the exact number of moles, this is where is end up:
200 = 24,37n (24 - 0,0388n)-1 - 0,00152n2

So please enlighten me, how can I proceed to calculate the value for n?

2. Jan 19, 2010

### Char. Limit

First, remember that the Van Der Waals pressure doesn't have to match the ideal pressure: in fact, it won't, unless a is 0. With an a as large as yours, the pressure can actually have quite a bit of variation between ideal and Van Der Waals.

For that last equation, have you considered quadratics?

3. Jan 20, 2010

### ehild

You can apply some iteration procedure. Rewrite the Van der Waals equation in the form

$$n=\frac{PV}{RT}(1+\frac{n^2a}{PV^2})(1-\frac{nb}{V})$$

Plug in the data, start with n you got for the ideal gas, substitute for n at the right side, and you get a new n. Continue the procedure with the new n... You get a value near 174 mol, if I am not mistaken.

ehild

4. Jan 20, 2010

### larsb

Thanks, I know that it doesn't match the actual pressure and the ideal pressure do not have to match...
I know for sure that the pressure in the cylinder is 200 bar, or actually 201 bar, because I am reading the pressure from a gauge.

What do you mean by "considering quadratics"?

5. Jan 20, 2010

### larsb

Ok, that indeed helps me a lot! I do not have to be completely spot on, however the closer the better.