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Probably and obvious chemistry question

  1. Apr 1, 2010 #1
    Hi everyone,

    I'm not much good at chemistry, but I'm in need of a bit of help for a project I'm doing.

    My question is: is it possible to convert a volume of helium into a volume of hydrogen? I'm looking at measurements of percentage of helium in the air, and want to convert them to an equivalent for hydrogen.

    Sorry if this is a stupid question, I really am bad at chemistry.

    Thanks for any help you can give me!
     
  2. jcsd
  3. Apr 1, 2010 #2

    Borek

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    Staff: Mentor

    Could be it is not a stupid question, but it doesn't make much sense as worded. You know, it is not even wrong. I guess you are mixing several ideas and concepts trying to solve some not well defined question. Or the question is well defined, it is just that you are confused and your approach is off.

    Please elaborate on the problem you are trying to solve. What kind of equivalence between hydrogen and helium are you looking for?

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  4. Apr 2, 2010 #3
    Ok I'll give it another go. It is quite an abstract idea though as I don't even know where to start, I only went up to Chemistry GCSE so barely have a grasp on moles haha.

    Basically I am looking into the safety of hydrogen fuelled vehicles. Long story short, the amount of hydrogen that leaks into the passenger compartment after an impact needs to be measured to make sure the amount isn't going above the lower flammability limits (4%).

    But hydrogen is too flammable to use in a normal crash test, so it needs to be done with helium filling the tanks instead.

    So my thinking is, there needs to be a helium sensor in the passenger compartment. It will give me a reading of the concentration of helium present (in ppm or maybe mg/kg) and once they give a reading, that reading needs to be converted into one for hydrogen.
     
  5. Apr 2, 2010 #4

    Borek

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    Staff: Mentor

    If you have it in mg/kg just divide by two. Thats from simple comparison of molar masses.

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  6. Apr 3, 2010 #5

    Char. Limit

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    Gold Member

    But isn't the volume of all gases the same... irrespective of mass?
     
  7. Apr 3, 2010 #6

    Borek

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    Staff: Mentor

    Molar volume, not volume per mass. 1 mole of hydrogen has the same volume as 1 mole of oxygen, 2 g of hydrogen have the same volume as 32 g of oxygen.

    Note that this simplified approach will work on ppm level, when the concentrations are much higher calculations can get a little bit more difficult. Still, mg/kg is a very good starting unit.

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    methods
     
  8. Apr 5, 2010 #7

    epenguin

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    Homework Helper
    Gold Member

    Just brainstorming - sure He is twice as dense as H2. But this is a problem of the leakage or injection under pressure of a light gas into a heavier one and wouldn't the dynamics of mixing depend mostly on the difference of densities between the light gas and the dense one (air more than 14 times as dense as hydrogen) so making the helium behaviour not a bad model for hydrogen behaviour?
    However if so I imagine there is some engineering experience and practice around it so maybe an engineering thread might find someone who knows what he is talking about? :biggrin:
     
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