Here a book says that Berzelius measured atomic masses using a simple lab and his measurements was so close to modern results. Can someone explain how a person living in 19th century was able to measure atomic masses? Like he took this and those, observed this and that and did something else. Can I repeat his experiments at home too and get his results?

Do I actually understand atomic mass right? For me it is a mass in gramms of one atom of an element.

Also, I can't understand how Gay-Lussac measured the volumes of gases reacted with each other. What does the book mean by saying 2 volumes of Hydrogen? How much are 2 volumes? Are they measured in m^3 or what?

Please try to avoid the use of moles in answers as I don't understand why we need moles actually. So, I think I need to understand questions above first before moving to moles.

  • $\begingroup$ Related: How was it possible to accurately measure the mass of gases in the 19th century? Actually, I think it is an exact duplicate. Could Turkhan please confirm? Welcome to Chem.SE btw! $\endgroup$ Jul 3, 2018 at 14:09
  • $\begingroup$ I guess two volumes of hydrogen are twice the volume of another gaseous reactant. $\endgroup$
    – aventurin
    Jul 3, 2018 at 16:10
  • $\begingroup$ Many people seem to believe that people were stupid in the old days. One of my favorite counterexamples is from 1932. It was the early days of nuclear physics (the neutron had just been discovered), and Cockroft and Walton used their newly-invented accelerator to drive the $^{7}$Li(p,$\alpha$)$\alpha$ reaction - the first human controlled nuclear reaction. Based solely on the observed range of the $\alpha$ particles they concluded a reaction Q valued of 17.2MeV. The current accepted value is 17.3468MeV. People were smart 100 years ago. $\endgroup$
    – Jon Custer
    Jul 3, 2018 at 18:02
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    $\begingroup$ @JonCuster, why I ask this is not to understand how stupid/clever they were. Instead, I want to understand why I am not able to conduct such experiments and make such measurements $\endgroup$ Jul 3, 2018 at 21:59
  • $\begingroup$ Perhaps a quote from the book would be helpful here. If you're not a chemist, your paraphrasing of the book may be incorrect. A better question is why are you bothering to repeat this? First, these values are well known, and second, you should not attempt to conduct experiments at home without knowledge of chemistry. No, moles are much more important than measuring atomic mass. $\endgroup$
    – Zhe
    Jul 3, 2018 at 22:21

1 Answer 1


Careful measurements were the foundation of chemical theory

Let's take the questions out of order to simplify the answers.

It doesn't matter what yours you use to measure volume as long as you stick to the same units in all your calculations. Cubic widgets is fine as long as you do all your calculations using them.

The key to understanding how chemists came to know about atomic mass is careful measurements of the widths and volumes involved in reactions. Chemists observed that certain reactions always involved the same mass ratios. More interestingly, when gases are reacted they also involve the same volume ratios. When hydrogen and oxygen react to for water you start with two volumes (2 cubic widgets if you want) of hydrogen and one of oxygen react together to give water. How do we know it is just that ratio? Do the experiment with 3 volumes of hydrogen and one of oxygen and observe that there is still one volume of hydrogen left after the reaction is done.

Those experiments showed that the mass ratios were constant but different to the volume ratios if the reactants were gases. Oh, and we know the weight of the gases because we can measure the weight of a given volume of gas using a vessel of known volume containing a gas at a specific pressure using a carefully constructed vessel. A litre of oxygen weighs about 1.5g which is easy to measure using a good balance and good experimental conditions even in the 19th century.

Chemists hypothesised (and later proved) that the volume of gases were proportional to the number of molecules they contained so the ratio of gas volumes told them something about the number of units of substance (or molecules) involved in reactions. Comparing many reactions led to an understanding of more complicated reactions than hydrogen burning in oxygen. Comparing the known results from gas volume ratios to the known weights of substances involved in liquid and solid reactions extended this idea (with an early glitch caused because chemists didn't realise that many gases consist of diatomic molecules).

The actual number of molecules or atoms in a give volume took a lot longer to determine than the basic principles involved in chemistry (that reactions always involved specific ratios of inputs and outputs). Initial methods involved atomic theory applied to gasses or measuring the charge on a single electron. Later methods involved measuring the mass of a very pure crystal and comparing it with density derived from x-ray measurements to give more accurate values. (see this).

The basic experiments about atomic ratios can be repeated with good experimental technique and apparatus. But you need a very good way to measure weights of gases and volume. Repeatable, but not easy. You won't be easily able to measure the number of atoms in anything with household apparatus, though. That requires fairly sophisticated equipment.

  • $\begingroup$ How they were able to put the required gas into cubes? Just imagine I am at home. How can I put all neded gases into some cube knowing that the cube contains exactly the gas I need assuming I am in 18-19th centuries? $\endgroup$ Jul 3, 2018 at 23:40
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    $\begingroup$ @TurkhanBadalov Vessels have fixed volumes even if they are not cubes. And that volume is easy to measure. $\endgroup$
    – matt_black
    Jul 3, 2018 at 23:42
  • $\begingroup$ okay. But how do I collect the required gas? How did they obtain all required pure gases, especially hydrogen? $\endgroup$ Jul 3, 2018 at 23:44
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    $\begingroup$ @TurkhanBadalov teaching chemistry from scratch (which is what you are effectively asking for) is a little beyond the scope of a single answer here. $\endgroup$
    – matt_black
    Jul 4, 2018 at 13:04
  • $\begingroup$ you mean it is a large topic how to fill a vessel with a gas assuming we are in 18-19th century? I will be thankful if you share some link related to this $\endgroup$ Jul 4, 2018 at 15:10

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