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    Hi,

    I have in my notes:

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

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

    \frac{\rho X_i}{A_i m_H}

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

     \rho = \frac{m_t}{V}; X_i = \frac{m_i}{m_t}; A_i m_H = m_i

    so then

     \frac{\rho X_i}{A_i m_H} = \frac{m_t}{V}\frac{m_i}{m_t} \frac{1}{m_i} = \frac{1}{V}

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

    Thanks
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    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)
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    (Original post by Bradshaw)
    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, N_i A_i m_h 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 X_i then, which should be X_i = N_i m_i in which case I get \frac{\rho X_i}{A_i m_H} = \frac {N_i}{V} which is the number of particles of element i per unit volume q.e.d!
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    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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