At class, we were (briefly) taught about the Loschmidt Number/Constant ($n_0$), which isn't really different from the Wikipedia definition,

The Loschmidt constant or Loschmidt's number (symbol: $n_0$) is the number of particles (atoms or molecules) of an ideal gas in a given volume (the number density).

Furthermore, it says,

It is usually quoted at standard temperature and pressure, the 2010 CODATA recommended value is per cubic metre at $0 °C$ and 1 atm.

(Yes, I know STP is currently defined at 1 bar pressure, and not 1 atm. Since this isn't my main question, I'm bringing it up here: Is the Wikipedia definition of the Loschmidt number inaccurate? Should it be defined at 1 bar, instead?)

Reading on, I saw,

Being a measure of number density, the Loschmidt constant is used to define the amagat, a practical unit of number density for gases and other substances:

1 amagat = $n_0$ = $2.686 7805(24)$ x $\mathrm{10^{25}}$ per cubic meter,

such that the Loschmidt constant is exactly 1 amagat.

To begin with, I've never heard of an "amagat" [<--- I mean the unit, not the law named after the same person]. Yes, this may be the result of my shortcomings as a high-schooler with limited (almost pitiful, really) exposure to physical chemistry...but what bugs me more, is that I still can't wrap my head around it even now.

Including the Wikipedia definition for the amagat before I continue,

An amagat is a practical unit of number density. Although it can be applied to any substance at any conditions, it is defined as the number of ideal gas molecules per unit volume at 1 atm and 0 °C.

We already have the good ol' Loschmidt Number...so why introduce another term/unit that is (seemingly) defined the same way, has the same value and carries the same (SI) units?

Besides, what's with the "...practical unit of number density." bit?


1 Answer 1


I haven't seen any of these particular units before so I could be mistaken, but I imagine it is similar to how we don't say "I have 3 Avogadro's Numbers of water" we would say "I have 3 moles of water". We very well could describe amounts by explicitly naming multiples of the constant, but we instead defined a new unit so that we could describe things more easily.


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