chemistry question?

In unit of gas laws and Dalton's law. This is based on a book problem. But changed the numbers. The names don't really matter.

PROBLEM: We have a methane and oxygen mixture (CH4 + O2) - just made this up btw. 

- Partial pressure of methane: 180 mm Hg (again made this up)

- Partial pressure of oxygen: 540 mm Hg (again made this up)

QUESTION: What is the ratio of the number of moles of methane to the number of moles of oxygen?

Tbh, this seems simple. Answer key just says 180/540, so 1 mole of methane for every 3 moles of oxygen which can be a helpful conversion factor to solve another stoic problem or something, but conceptually this doesn't make sense to me.

HERE"S WHY I"M CONFUSED: Shouldn't oxygen, with a much lower partial pressure indicate a greater volume (Boyle's Law), which would mean that the number of moles should be higher, as "n" is directly proportional to "v." ??

Since the problem mentions nothing about the volume of the tank, should I assume that the volume is constant? I just thought higher pressure = less volume = less moles, but I guess it doesn't matter if volume is constant, right? Then I guess I understand the ratio. Sorry for the ramble! 


Oh, could it have to do with the equation:

P total = n total (RT/V)

just for partial pressure tho.?

1 Answer

  • 1 month ago
    Favourite answer

    The two gases are contained in the same container ("a mixture") and so the volumes of the two gases are the same. So, using Boyle's law doesn't work. 

    Avogadro's Law says that if two gases have the same volume, temp and pressure, they contain the same number of particles. From this, it makes sense that if temp and volume are the same, the pressure will be proportional to the number of particles (or moles). More moles, more pressure. 

    The relationship is: P1/n1 = P2/n2. You can derive this from the ideal gas law where T and V are constant.

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