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Thread: Empirically figuring out atomic mass

  1. #1 Empirically figuring out atomic mass 
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    So historically, when people were first working out the relative masses of each element how did they go about doing it in a lab?

    I read somewhere that they'd see you need like 16x as much O2 to react with a given mass of H2 therefore O is 16 times as heavy as H. But at that time how could they have known that oxygen and hydrogen were diatomic? how do you work that out?

    many thanks


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  3. #2  
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    They started by weighing the products of reactions. That gave them relative mass numbers.
    They discovered that some of what they thought were simple single atoms of elements were actually molecules of the element through the same method. Don't forget there are a lot of reactions besides H2O like H2O2 making hydrogen peroxide.
    Dalton was the one who did the most work on determining the atomic weights and he was the one who proposed using Hydrogen's weight as the basic unit to compare all the other weights to.
    That is why Hydrogen is one.


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  4. #3  
    Bullshit Intolerant PhDemon's Avatar
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    What Dan said is correct but modern mass numbers are defined relative to carbon-12.(which is defined as 12 obviously).
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    What I said was incomplete though, and I will likely need some help on this part because it was a long time ago that I was taught it.

    There was another guy who figured out that all gases had the same number of molecules in them at the same volume and pressure. His name was Boyle.
    After him comes a guy called Avogadro who realised that any gas filling 22.4 liters of volume at atmosheric pressure weighed the same in grams as what the atomic number of the atoms in the molecules of gas was. He was not actually able to give a number of atoms though so it was only later that some guy called Loschmidt came up with an actual number.
    6.022x1023

    That is about all I remember about it.
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  6. #5  
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    One thing to recognise is that when the proportions by weight of the various elements in many compounds are compared, integer relationships manifest themselves which enabled chemists to establish chemical formulae and relative molecular weights. In the beginning, mistakes were made, but with the accumulation of more data and technological advances, these mistakes were eventually corrected. Nowadays, chemists have available to them very powerful tools for determining chemical structure.
    There are no paradoxes in relativity, just people's misunderstandings of it.
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  7. #6  
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    Quote Originally Posted by dan hunter View Post
    What I said was incomplete though, and I will likely need some help on this part because it was a long time ago that I was taught it.

    There was another guy who figured out that all gases had the same number of molecules in them at the same volume and pressure. His name was Boyle.
    After him comes a guy called Avogadro who realised that any gas filling 22.4 liters of volume at atmosheric pressure weighed the same in grams as what the atomic number of the atoms in the molecules of gas was. He was not actually able to give a number of atoms though so it was only later that some guy called Loschmidt came up with an actual number.
    6.022x1023

    That is about all I remember about it.
    Yes I seem to recall gas density came into this, i.e. weighing 22.4l of various gases at NTP and looking at the ratios of the results. But my recollection is hazy.
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