Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

How did the chemists in the early days

  1. Feb 5, 2009 #1
    How did the chemists in the early days measure the atomic mass of the elements that were discovered up to that point? This question is urgent since I asked my Chems teacher this question and she said "You know what," and she was laughing "why don't you go research this for me and bring it back tomorrow so we can discuss it in class":surprised

    And you can't say no to the teacher...

    Note: This is not a homework question
  2. jcsd
  3. Feb 5, 2009 #2


    User Avatar
    Science Advisor
    Homework Helper

    Ratios, even today there is no need in chemistry to really know how many atoms per gram, so long as you know the relative masses you can work out the proportion of each element in a compound and so t's formula.

    If you want to know the actual number of atoms/gram look up Avagadro's number
  4. Feb 8, 2009 #3


    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    Last edited by a moderator: Apr 24, 2017
  5. Feb 10, 2009 #4


    User Avatar
    Homework Helper
    Gold Member

    Good question.
    Not so easy surely, you get from proportions 'equivalent masses' and it was a bit of a detour before they could be confient of atomic masses. E.g. Dalton thought water was HO.
  6. Mar 2, 2009 #5
    I like your chemistry teacher -- you asked a GREAT question, and she asked you to find out. Hope you did, but if not, here're some thoughts.... (and tell her she's a GREAT teacher... ;-)

    Start with PROPORTIONS. Early chemists figured out that you'd mix 1 part of X with 2 parts of Y. Let's say we're talking about water (H2O) -- and let's just pretend that we live on a world where we can weigh some hydrogen or oxygen gas as easily as we weigh water or flour, mmm-kay?

    Now - we know that water is 1 part oxygen, and 2 parts hydrogen. How? We split water with electricity, collect the gas, and measure the volume. According to Boyle's law (or the ideal gas law, etc. -- hey -- I'm not going to feed you EVERYTHING!), if you've got the same number of molecules of gas -- ANY gas! -- at the same temp & pressure -- you've got the same VOLUME.

    So - you electrolyze some water, you get two gases, one volume of which is EXACTLY (as near as you can measure) double the volume of the other. So - you hypothesize that the first gas (hydrogen) exists in water in exactly 2x proportion to the other (oxygen).

    Now you weigh them. Hydrogen is the lightest thing you've ever found, so let's just assign it a weight of "1". (Doesn't matter what the units are -- carats per bushel is fine -- the only thing that matters is the relative weights -- which actually should be masses, and in this case are, but never mind. >;-) You weigh the amount of Hydrogen, then the equivalent amount of Oxygen, and find that, after adjusting for the 2x number of "units" (in this case, either atoms or molecules, as both H and O form diatomic molecules) -- that the Oxygen is 8x more massive than the Hydrogen.

    Bingo. Hydrogen: atomic weight (arbitrary): 1. Oxygen: 8 x H.

    (We are NOT going to get into why these numbers are off by fractional percentages -- your question was about the EARLY chemists. Until we introduce isotopes, quantum mechanics, neutrons, etc. -- let's just use nice, whole round numbers... ;-)
  7. Mar 2, 2009 #6


    User Avatar
    Science Advisor

    Ah but corradini, you have the benefit of hindsight, so you know what emphasis to place on the Ideal Gas Law. The chemists of the early 19th century were not as clear on the matter.
    Back then, they mostly fooled around with 'classical' elements like C,N,O, P, S. Which all happen to be ones with an mass that's roughly 2Z. Wouldn't it be convenient then, to ignore that pesky hydrogen gas pressure and assume that C, N, O etc had the neat weights of 6,7,8 and hydrogen had 1. How could that be? Well, hydrogen happened to occur in pairs a lot in the molecules they worked with. H2, H2O, H2SO4, H2CO3, etc. So they assumed instead that it was H and HO etc.

    It took decades before they'd finally sorted it out that if H was 1 then C was 12 and so on as we know now, together with the correct stoichiometric formulas.

    The Swedish chemist Berzelius (1779-1848) was without a doubt the Great Master of element measuring. (and he discovered quite a handful as well) Some of his measurements weren't surpassed in accuracy until well into the 20th century! But he did spend his life being off by a factor of exactly 2! :)
  8. Mar 2, 2009 #7
    So - excellent points, alxm. (Clever nym, by the way...)

    1) I'm not really a chemist - I don't even play one on TV. I'm a burned out software marketing VP, who's read a LOT of chemistry lately, had a great idea and spent 2+ years in my garage building reactors, and recently raised a bunch of money and hired (among others) a PhD in ChemEng who tells me what's wrong with my thinking.

    But - we're doing pretty well. ;-)

    2) I'm guessing there're a few H compounds in which hydrogen occurs singly, no? Like, um, HCl? KOH? NaOH? All of which were of considerable interest during the few hundred years of European internecine warfare.....and thus considerable incentive to understand.... And, if you do the Boolean/whole-number math....

    Just a thought. I could be wrong -- I usually am.

  9. Mar 2, 2009 #8


    User Avatar
    Science Advisor

    Certainly there are single-hydrogen compounds! Although KOH and NaOH are pretty good examples of the hurdles Nature set up for the early chemists. Before electrochemistry (circa 1810 or so) nobody had ever even seen pure, metallic K or Na. And chlorine is a nasty isotope mix with about 25% Cl-37 giving it an essentially half-integer weight. It was all pretty messy.

    So it was indeed a subject of great debate. And it was settled by HCl, in fact! The Italian Cannizzaro used the weight of hydrogen in HCl as the base of his system in 1858. (That is, _one_ atom of hydrogen). All the great chemists of the day got together at a congress in Karlsruhe in 1860 to sort it all out.

    Cannizzaro's convinced everyone of his system. So that's when the modern system of atomic weights came into play, together with proper definitions of 'atomic weight', 'formula' versus 'empirical formula', and not least, the consensus that Dalton was correct and stuff was made out of atoms. (A consensus among chemists at least. It took another half-century before all physicists were on board)
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook