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In chemistry, the unit "mole" is used in many places, and is an important tool when working with quantities.

I have not seen it used elsewhere--for example, we never count people or TVs with the unit.

What makes the mole so useful in chemistry, and why isn't it used elsewhere?

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Because the mole is not a convenient basis for people and TV sets (for instance, one mole of people would be approximately 86 trillion (short-scale) times the population of earth, or put another way the earth contains a human population concentration of approximately 12 femtomoles/planet), and because there's no connection back to atomic mass, which is what makes the mole useful in the first place.

This was not part of your original question, but addresses Manishearth's modifications:

Because atoms and molecules react in discrete units (cf. the law of definite proportions), the number of particles is more fundamental than the masses of those particles. The mole is useful in chemistry because it is defined such that the mass of Avogadro's number of particles (atoms or molecules) of a substance (the molar mass) is equal to the sum of the numerical values of the atomic masses of its constituent elements. The value of the mole is calibrated such that the molar mass of carbon-12 is exactly 12 grams per mole. On the periodic table you will note that the atomic masses given are non-integral (often markedly so, e.g. chlorine at 35.45 g/mol). This is because the atomic masses represent a weighted average of isotopic masses, and because the proton and neutron masses aren't quite equal.

The mole makes stoichiometric calculations particularly convenient as we can measure out, for instance, 16 grams (one mole) of $\ce{^{16}O}$ and 2 grams (two moles) of $\ce{^1H}$ and obtain one mole of $\ce{H2O}$. In the case of water, oxygen makes up 89% of the molecule by mass, but is only one of the three atoms present in the molecule. The mole is necessary to transpose stoichiometries involving single atoms and molecules (with their tiny masses) into the everyday world of grams and litres.

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    $\begingroup$ Yep, would've been a shame to see this go to waste.. :) $\endgroup$ – ManishEarth Jun 21 '12 at 15:53
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    $\begingroup$ $8.6*10^{13}$ more than earth's population?! Heh, it humors me that I always thought Avagadro's as so big as to be incomparable to anything. Quantifying with 86 trillion times earth's population is suddenly even more absurd. Thanks for the chuckles. +1 $\endgroup$ – Chris K Jun 21 '12 at 18:39
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    $\begingroup$ @ChrisK - Well, there are about 4.9 octillion IPv6 addresses for every human being, which makes Avogadro's number look positively miniscule. $\endgroup$ – Richard Terrett Jun 22 '12 at 7:04
  • $\begingroup$ A mole of stars would be all the stars in all the galaxies we can see, and more. $\endgroup$ – DarenW Oct 31 '12 at 15:51
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    $\begingroup$ To satisfy my curiosity, I calculated the length for a mole of dimes in a stack. It would just about be the diameter of the Milky Way. $\endgroup$ – LDC3 Dec 12 '14 at 6:22
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The Si-unit "mol" is used in chemistry for three reasons:

  1. The quantity of atoms/molecules is the logical unit of chemistry (Reactions have to be balanced).
  2. Atoms/molecules can't be counted (It works for people and TV's), but have to be measured using mass or volume, and then converted into a quantity (e.g. mol/kg)
  3. The quantity of atoms/molecules involved in a typical experiment or measurement is very large, making a shorthand useful (Compare this to the Astronomical Unit which is a shorthand for a large number of km).

The mol (or a similar unit) isn't used elsewhere because: Anything larger than chemicals can be measured with whichever measure seems to be the most logical. Chemistry is unique in requiring chemical quantities to make sense of the world but at the same time these quantities can not be determined directly.

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Apparently mol is huge unit and that makes in not very useful for normal life, however there is another simple reason: there is no balance or cylinder that can measure mol. The unit has its logic in sense of reactions (equimolar etc.) but still every single experimental chemist has to start with weight or volume when he prepares his synthesis.

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The mole is useful in chemistry because it relates two popular but inconsistent units of mass - the atomic mass unit (u) and the gram. One mole of atoms of an element with atomic mass x u, has a mass of x grams.

We don't use atomic mass units outside chemistry, so we also have no need for the mole.

This of course raises another question "why do chemists use atomic mass units?". Perhaps it's because they measure quantity in moles :P Really I think it's just an accident of history from the days when people thought atomic masses were all integer multiples of the mass of hydrogen.

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