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Topic: Calculate density  (Read 5213 times)

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Offline professordad

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Calculate density
« on: August 19, 2012, 02:05:03 PM »
The mineral uraninite (UO2) adopts a fluorite structure in which the length of an edge of the unit cell is 5.468 Angstroms.  Calculate the density of uraninite.
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Offline Dan

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Re: Calculate density
« Reply #1 on: August 20, 2012, 02:53:50 AM »
Show your attempt. Forum Rules.
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Offline professordad

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Re: Calculate density
« Reply #2 on: August 22, 2012, 10:41:06 AM »
Right sorry..

Ok so uraninite has an atomic mass of 238.02891 amu + 31.9988 amu = 270.0277 amu.  Multiply this by 1g / (6.02·1023 amu) and we get 4.49·10-22 g.  This is the mass.

For the volume, it's just 5.4683 · 10-24 cm3, or 1.635·10-22 cm3.  

So the density should be 2.75 g/cm3 if we divide thru.

But the answer given is 10.97 g/cm3...

edit: fixed a few errors
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Offline curiouscat

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Re: Calculate density
« Reply #3 on: August 22, 2012, 11:30:30 AM »
Is the molecules / Unit Cell = 1?
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Offline curiouscat

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Re: Calculate density
« Reply #4 on: August 22, 2012, 12:36:50 PM »
Quote from: curiouscat on August 22, 2012, 11:30:30 AM
Is the molecules / Unit Cell = 1?

If species/UC  be ~4 you'd get the desired answer...Is it?
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Offline XGen

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Re: Calculate density
« Reply #5 on: August 22, 2012, 05:28:03 PM »
Can you describe to me what the fluorite structure is?
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Offline AWK

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Re: Calculate density
« Reply #6 on: August 22, 2012, 05:34:06 PM »
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AWK

Offline professordad

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Re: Calculate density
« Reply #7 on: August 23, 2012, 11:13:20 AM »
The fluorite structure was something mentioned earlier in the book; I forgot to explain it.  According to AWK's link it should be AB2 - all tetrahedral - and CAF2 (the example given in my book) describes its structure.  Hopefully this clears things up.
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Offline fledarmus

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Re: Calculate density
« Reply #8 on: August 23, 2012, 01:32:51 PM »
Okay, looking at their pictures of the unit cells, how many molecules of UO2 are contained within a unit cell? Once you know how many molecules are in the cell, since you have already calculated the weight of each molecule and the volume of each unit cell, you will be able to calculate the density.
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