# What-where is the proof for Avogadro΄s law?

## Main Question or Discussion Point

That 1 litre of hydrogen contains the same number of molecules with 1 litre of oxygen.

because in experiments, 2 litre hydrogen gas combined with 1 litre oxygen gas .
and by chemical reaction we know 2 H2 + O2 ----> 2 H2O
so 2 litre hydrogen contains double number of atoms than 1 litre oxygen ( Provided other conditions like pressure temperature of both are same)

Can you post links of this evidence? I have my doubts you see. E.g. when you say "1 litre of hydrogen" you mean that they let a particular weight of liquid hydrogen to evaporate in a box of 1 litre which contained vacaum, and also etc?

Last edited:
.Scott
Homework Helper
Can you post links of this evidence? I have my doubts you see. E.g. when you say "1 litre of hydrogen" you mean that they let a particular weight of liquid hydrogen to evaporate in a box of 1 litre which contained vacaum, and also etc?
http://www.engineeringtoolbox.com/gas-density-d_158.html
Note these STP densities:
$O_2: 1.4290 kg/m^3$
$H_2: 0.0899 kg/m^3$
And these molecualr weights:
$O_2: 32.000$
$H_2: 2.016$
Dividing through:
$O_2: 1.4290 kg/m^3 / 32.000 = 0.0447 kg/m^3$
$H_2: 0.0899 kg/m^3 / 2.016 = 0.0446 kg/m^3$

DUDE, this density of hydrogen is derived by Avogadro΄s hypothesis? (Not to mension the desity of oxygen). I've seen the link, where is the experiment you said. Do you feel brainwashed and you cannot see it as a hypothesis, or you are 100% certain it΄s prooven. WELL, I HAVEN΄Τ SEEN THE PROOF, WHERE IS IT. I am tided of guessing proofs. ALSO, if we have weight of 70 kilograms of liquid hydrogen in 1 metre^3 I am not sure that its mass i.e. the number of its noucleons is (70/1000) (the number of nucleons in 1000 kilograms of water), because as this is a low density comparing to water΄s, I am not sure whether e.g. any surrounding buoyncy because of air is taking place (and I am not sure of the density of air yet). THUS, something at electrolysis or counter-electrolysis can be the only proof I can think of.

mfb
Mentor
It is not necessary to get rude, or write in caps.
this density of hydrogen is derived by Avogadro΄s hypothesis?
No, it is an experimental result. You weight a box (with known volume) with vacuum, and weight it filled with hydrogen at standard pressure and temperature, and calculate the difference. No liquids involved.
The same experiment can be done with all other gases. Avogadro's law is not exact, but it is a really good approximation at standard conditions.

because as this is a low density comparing to water΄s, I am not sure whether e.g. any surrounding buoyncy because of air is taking place
That's why you compare it to a box with vacuum. Or, once you measured the density of air that way, you can subtract it via calculations.

THUS, something at electrolysis or counter-electrolysis can be the only proof I can think of.
That would be equivalent to the chemical reaction suggested above. You always get twice the volume of hydrogen compared to the volume of oxygen in water electrolysis, for example.

PeterDonis
Mentor
2019 Award
Can you post links of this evidence?
Avogadro's hypothesis was tested by extensive experiments in the 19th century; it has been accepted fact since then, which is why it's now called Avogadro's Law (not hypothesis).

if we have weight of 70 kilograms of liquid hydrogen in 1 metre^3 I am not sure that its mass i.e. the number of its noucleons is (70/1000) (the number of nucleons in 1000 kilograms of water)
Avogadro's Law applies to gases, not liquids.

Doug Huffman
Gold Member
To show an example of Avogadro's Law being correct, you must show that V1/n1=V2/n2 (at STP). The number of moles of gas can be shown with experiments, as previously stated, and the volume at standard temperature and pressure is simple: just put in a piston to the outside, and cool the container to 0 degrees Celsius. For proof, well, it can be proved from the Ideal Gas Law.

Quantum Defect
Homework Helper
Gold Member
http://www.engineeringtoolbox.com/gas-density-d_158.html
Note these STP densities:
$O_2: 1.4290 kg/m^3$
$H_2: 0.0899 kg/m^3$
And these molecualr weights:
$O_2: 32.000$
$H_2: 2.016$
Dividing through:
$O_2: 1.4290 kg/m^3 / 32.000 = 0.0447 kg/m^3$
$H_2: 0.0899 kg/m^3 / 2.016 = 0.0446 kg/m^3$
Units in the last line are incorrect. 0.0447 kmol/m^3 for O_2 and 0.0446 kmol/m^3 for H_2. You can do the same calculation for any gas (use mass density and molecular mass to determine the number density). Most gases under "typical" conditions give comparable values. There are real differences between gases, which have to do with the non-ideal behavior of the various gases (molecules have finite size, molecules have non-zero intermolecular forces).

epenguin
Homework Helper
Gold Member
Can you post links of this evidence? I have my doubts you see. E.g. when you say "1 litre of hydrogen" you mean that they let a particular weight of liquid hydrogen to evaporate in a box of 1 litre which contained vacaum, and also etc?
Look up Gay-Lussac's law of combining volumes which is experimental.
Avogadro's hypothesis was invented to explain it.
Also the kinetic theory of gases explains it.
I am not sure of the history around this but I think although a primitive form of kinetic theory (Bernoulli) existed before Avogadro it played no role in the thinking of the chemists and the useful empirical laws only got tied up with theory by Maxwell and Boltzmann, quite late. But I would like to be referred to books or other sources for the history of this.

You are right IMHO to suspect circular reasoning. In fact I think #2 and #4 for instance contain circular reasoning

Everything is by now woven together in a confident final explanation, some steps of the ladders it was arrived at by have been kicked away.

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PeterDonis
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2019 Award
I think #2 and #4 for instance contain circular reasoning
How so?

PeterDonis
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2019 Award
when you say "1 litre of hydrogen" you mean that they let a particular weight of liquid hydrogen to evaporate in a box of 1 litre which contained vacaum, and also etc?
No, he means a one litre vessel was filled with hydrogen gas at a particular temperature and pressure. (Actually, in his example, it was two litres of hydrogen and one litre of oxygen.)

epenguin
Homework Helper
Gold Member
How so?
One quotes atomic masses and the other a formula both assumed as known. How were they known? Originally via Avodgadro's hypothesis I think. At the start all anyone had was combining weights, so for Dalton water was HO, and the atomic mass of oxygen would have been 8.

mfb
Mentor
How were they known? Originally via Avodgadro's hypothesis I think.
Right, but now we have mass spectrometry for a direct measurement.

PeterDonis
Mentor
2019 Award
At the start all anyone had was combining weights, so for Dalton water was HO, and the atomic mass of oxygen would have been 8.
Yes, until other experiments were run with other chemical reactions that made it clear that the hypothesis that water was HO would not work and that the atomic weight of oxygen had to be 16, not 8. Avogadro's Law was not "proved" in a single blinding flash; it was gradually established over time as more and more data came in. And, as I pointed out in an earlier post, all this happened more than a century ago, and Avodagro's Law has been accepted fact ever since.

Any "evidence" anyone is going to post in this thread is obviously going to be just a tiny sliver of all the evidence that convinced chemists over the course of decades. That doesn't mean it's circular reasoning to just give that tiny sliver instead of quoting all the evidence and history at length; to me it's just a friendlier way of saying "here's the basic idea, go look up the details yourself".

epenguin
Homework Helper
Gold Member
Right, but now we have mass spectrometry for a direct measurement.
That's cheating. We are not now, we are in 1810 or several decades thereafter.

Alternatively it's a good example of what I said - kicking away the ladder we climbed to get to that position.

Or you could say that this hypothesis a after being used for 100 years for all the calculations the OP is probably starting to do some of, was finally proved around 1920. Though I doubt that is what they considered they'd achieved.

However I will settle for what PeterDonis says, and look forward to the enlightenment of QuantumDefect's link.