# How did Joseph-Louis Proust know what gases weighed?

Proust's work led him to conclude that compounds always contain the same ratio, by weight, of their constituent elements. How did he know (or deduce?) what gases weighed?

Precise answers appreciated - I can find nothing on the net that explains how the weight of gases was measured at that time - all accounts of Proust & Dalton's work skip over this & other accounts just refer to modern tech.

I am not a chemist - I just want to know this.

• Not that I feel this is off-topic here, but you may get a better answer over at History of Science & Mathematics Aug 28, 2017 at 18:15
• I think this should be migrated to History of science.SE Sep 28, 2017 at 9:57

He knew because he weighed them

This might seem ridiculously obvious but the early pioneers of modern atomic theory based the idea on actual results they got from weighing things before and after reactions.

It is perfect possible to weigh a container full of gas if you have an accurate enough balance (the buoyancy of the air cancels out if you use the same vessel for different gases). Note that the idea of a vacuum was developed in the 17th century so was well-known by the time of Proust and he could also have measured the empty weight.

The idea is simple: fill a vessel with some gas and weigh the vessel very accurately. A litre of oxygen weighs about 1.5g which is large enough to measure and with good equipment and technique large enough to measure accurately. A liter of hydrogen would be a little under .1g, a big difference.

A series of careful experiments like this with a range of gases would reveal something about the weight-ratios and carefully counting weights and volumes before and after reactions would tell you more (2 volumes of hydrogen react with one of oxygen, for example).

This is what people like Proust did, though with equipment we would now regard as primitive. The reason why it isn't mentioned a lot in modern descriptions is probably because it seems too basic and simple. Nowadays we just teach the theory not so much about the careful experiments that came first and actually led to the theory.

Disclaimer: I just remember this from my chemistry classes, when we talked a tiny bit about history. So I have no proper sources for when and with which gases these experiments were conducted.

First let's start with a very important chemical equation for your question. In a more or less good approximation gases obey the following relationship: $$pV=nRT$$ Or to put it in another way: $$V \propto n$$ If you double the number of particles in a gas, you get double the volume. What they could find experimentally is, that they needed e.g. 2 L of hydrogen to react with 1 L of oxygen in the burning of hydrogen to water: $$2 \ce{H2 + O2 -> H2O}$$

If 0.5 L of oxygen and 2 L were present, there were still 1 L of hydrogen remaining after the reaction. If this experiment was repeated for various gases, it was possible to see this recurring pattern of integer multiples.

Now comes the most important part of science: From these experiments they did not know for sure that there are atoms etc. At least not with the security of today where we can see them in AFM, mass spectrometry... BUT: The hypothesis of atoms and constant integer ratios in chemical reactions was in good agreement with the available experimental data of this time AND was short and elegant (Occam's razor).

And to answer your initial question. This does not require the knowledge about the molar mass. But starting from these findings, it was now possible to obtain the molar mass. On method was to vapourize known masses of a substance and apply the above relationship.