Why is amount of substance considered a fundamental quantity? [duplicate]

Question

The unit of amount of substance is mole. 1 mole is defined as the amount of substance that contains Avogadro Number (6.022 x 10^23) no. of entities in a substance. It is a fundamental unit.

However, in the measurement of mole, I guess it is measured with the help of another fundamental physical quantity, which is mass.

No. of moles= (mass of a substance) /(molar mass)

Definition of Fundamental Unit Fundamental units are those which are independent of any other unit.

Why is the ampere a base unit and not the coulomb? The most upvoted answer seems to explain that ampere can be measured with accuracy in lab but it's difficult to measure charge. In the same way, if I say, amount of substance can't be measured directly in labs, but can be measured with the help of mass, so amount of substance shouldn't be a fundamental physical quantity. But still it is. Why?

Why is the mole/"amount of substance" a dimensional quantity?

Here, the explanation lies mostly for why it's a dimensional quantity. Although there is hint for my question, I am still confused as my question is not only just about why it's a fundamental quantity but also why it will not be considered in the same way as it is in case of ampere being a fundamental quantity and not charge (link to the question already provided). Also I think mole and mass is related (already I have mentioned). So,it proves amount of substance is related to another fundamental quantity. Why is it still a fundamental quantity itself?

Answer 1

It is not true that the mole is defined in terms of another fundamental quantity (i.e. the kilogram). The current definition of the mole does not involve the kilogram or any other SI unit:

• "One mole contains exactly $6.022140 76\times10^{23}$ elementary entities."

The statement that "No. of moles=(mass of a substance)/(molar mass)" is true, but arguing that this means the mole depends on the kilogram is like claiming that the second is not a fundamental unit because "time = distance/speed".

Molar mass is defined as (mass of a substance)/(amount of substance). In other words, "molar mass" is not a fundamental quantity but is a secondary quantity that can be calculated from the fundamental quantities of mass and amount. The definitions of secondary quantities such as "molar mass" or "speed" can always be rearranged to give equations with a fundamental quantity on the left-hand side and a mix of fundamental and secondary quantities on the right-hand side.

It is true that the current definition of the mole was only adopted in 2019, in part to make it clear that the amount of "elementary entities" is independent of their mass. The former definition was that the mole was the number of atoms in 12 grams of carbon-12, which related the definition of the mole to the definition of the kilogram. This is no different, however, than how the definition of the metre depended on the definition of the second, since a metre was defined as distance travelled by light in vacuum in ⁠1/299792458⁠ of a second.

It is worth noting that there really is no such thing as a "fundamental unit", just fundamental constants. Quoting from the BIPM:

The SI is the system of units in which

• the unperturbed ground state hyperfine transition frequency of the caesium-133 atom Δν$_{Cs}$ is 9 192 631 770 Hz
• the speed of light in vacuum $c$ is $299 792 458$ m/s
• the Planck constant $h$ is $6.626 070 15 \times 10^{–34}$ J s
• the elementary charge $e$ is $1.602 176 634 \times 10^{–19}$ C
• the Boltzmann constant $k$ is $1.380 649 \times 10^{–23}$ J/K
• the Avogadro constant $NA$ is $6.022 140 76 \times 10^{23}$ mol$^{–1}$
• the luminous efficacy of monochromatic radiation of frequency $540 \times 10^{12}$ Hz, $K_{cd}$, is 683 lm/W

In this system, the mole actually seems to have the cleanest "fundamental" definition, since it only involves one fundamental constant (Avogadro's number). In contrast, the definition of the kilogram involves seconds, metres, Planck's constant, the speed of light, and the atomic spectrum of cesium.

Related Questions

Many people wonder why the mole exists, so there are many similar questions whose answers may be worth looking at:

• Why is the mole/"amount of substance" a dimensional quantity?
• Why is the mol a fundamental physical quantity?
• What are the reasons for making the mole a base SI unit?
• Is there a need for the unit mole?
• Why is the mole an SI unit?
• Why are Amperes and moles both base units?

From Chemistry SE

• Why is the mole a unit of measurement?
• Why do people still use the mole (unit) in chemistry?

Note that some of the questions and answers are from before the 2019 redefinition of the mole, so some of their discussion may no longer be completely relevant.