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# Mean Molecular Weight Watch

1. Hi,

I have in my notes:

Denote the atomic weight of element i as and its mass fraction as .

We have a gas containing a mixture of elements, and then the number of atoms of element i per unit volume is:

where is the mass of hydrogen. I can't figure out why this is the case. From what I see:

so then

Obviously something there is wrong. How do I get it to be equal to the volume density of the ith element?

Thanks
2. Bump
3. No one?
4. Two errors here, I think!

1) it's per unit volume so V=1
2) mi=Ni Ai mh (you need to multiply by the number of particles)
Two errors here, I think!

1) it's per unit volume so V=1
2) mi=Ni Ai mh (you need to multiply by the number of particles)
Mm, per unit volume doesn't mean V = 1. As in, a generic mas density is always defined as mass per unit volume and that means M/V. You divide by the volume to get the desired quantity per the smallest unit of volume, or time, or whatever.

Also, would be the total mass of all the ith element particles, not the mass of the of a single ith element particle.

That does actually clear up a mistake in what I wrote for then, which should be in which case I get which is the number of particles of element i per unit volume q.e.d!
6. Oops yeah you're right about the volume. For the other part, I was correct also (I dont know how you have defined mi!)

Anyways glad it seems to be cleared up now!

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Updated: April 19, 2013
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