# Mass of used hydrogen in an isothermal process

1. Jun 14, 2012

### Uku

1. The problem statement, all variables and given/known data

I have hydrogen in a 12 L tank, at T=15 C. Some of it is used, the T = const. and Δp=0.4 MPa,
the molar mass of hydrogen is M=2*10^-3 J/(mol*K). Find the mass of the used hydrogen.

2. Relevant equations

Am I wrong in simply using:

$\Delta m=\frac{M}{RT}V\Delta p$ ?

That gives roughly 4 kg's..

Thanks,
U.

2. Jun 14, 2012

### Infinitum

Hi Uku,

The equation you have used is appropriate. You could have gone wrong in the units, though.

3. Jun 14, 2012

### Uku

By "could have" you mean you put in the numbers and got a different answer?

4. Jun 15, 2012

### Infinitum

No, I didn't put in the numbers. I was just confirming the equation use

Though, 4 kg does seem a bit too much....

5. Jun 15, 2012

### ehild

The answer is different from yours. What number did you use for the volume? And the molar mass is 2*10-3 kg/mol, not J/(mol*K):tongue:

6. Jun 15, 2012

### Uku

Well the pressure differential is 0.4 MPa..

7. Jun 15, 2012

### ehild

The pressure is all right, but what number have you plugged in for volume?

ehild

8. Jun 16, 2012

### Uku

0.012 m3

9. Jun 16, 2012

### ehild

Well, then plugging in all data,
Δm=(2*10-3*0.012*0.4*10-6)/(8.3145*288)=9.6/2394 how can it be 4 kg?

ehild

10. Jun 16, 2012

### Uku

I messed up on R, since I wrote $R=8.314*10^{-3}\frac{kJ}{K}$ when trying to write R in terms of kJ.

11. Jun 16, 2012

### ehild

That I did not guess. :surprised Why in kJ-s?

But You got the correct result at the end now?

ehild