[smiler51]

Hi,

i am trying to create 2 different gas mixtures. If I understand Dalton's Law correctly, it mentions that P(Total Pressure) = P1 (Pressure of Gas# 1) + P2 (Pressure of Gas# 2) + P3 (Pressure of Gas# 3) + etc...

Now i am not really sure if this also applies when there are vast differences in the density of the gases, as for example when mixing Ar and H2, as this might change the gas percentages when we are talking about pressures in excess of 150bar. Does the density or pressure have any effect on the partial pressure calculations or does Dalton's Law always work as long as there aren't big temperature differences?

I want to make the following two mixtures:

the first mixture is 5% Hydrogen / 95% Nitrogen in a 40L gas bottle with a final pressure of 150bar. According to Dalton's Law, I would have to first fill the bottle with 7.5bar of H2 and add 142.5bar of N2 (7.5bar + 142.5bar = 150bar). Is this thinking and calculation correct?

The second mixture i need to make is a 3 component gas mixture in a 40L gas bottle with a final pressure of 150bar: 5% Carbon Dioxide / 40% Helium / 55% Nitrogen.
My approach would be to fill 7.5bar CO2 into the bottle then add 60bar of He to 67.5bar and lastly add 82.5bar of N2 to achieve a final pressure of 150bar. Is this thinking correct as well?

I appreciate the help.

Thanks & Best Regards,

Nail




 

[opti384]

Another thing about the Dalton's law and gas mixtures is the partial pressure of the gas is proportional to its mol fraction.

Mol fraction is the "mol of a substance" / " the sum of mols". For example, let's there are 2 mols of N₂, 3 mols of H₂ and 5 mols of O₂. Here, the mol fraction of O₂ will be 5/10= 0.5.

In your first example, if there is a mixture of 5% Hydrogen / 95% Nitrogen in a 40L gas bottle with a final pressure of 150bar, the partial pressue of hydrogen should be 63.6 barr and that of nitrogen should be 86.4. However, if we mix nitrogen with partial pressure of 142.5 bar and hydrogen with 7.5 bar, the mass composition ratio will not be 5% hydrogen / 95% nitrogen.

[smiler51]

Thank you very much Opti. Sorry for not replying earlier. I guess the proper way of calculating a gas mixture is through it's moles then. I will have to study this a bit more, as I am still not understanding it totally.

Thanks again

Smiler51