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We all have read that 1 mole of any substance contains $6.022 \times10^{23}$ atoms included in the substance. And: $$Mole=\frac{weight}{molar\,\,mass}$$

Where molar mass is gram atomic mass such that when the atomic mass of the element (expressed in unified mass) is expressed in grams, then that amount contains $6.022 \times10^{23}$ atoms of that element. For example for oxygen, ($unified\,\,mass=16u$) when we take $16$ grams of it, it has $6.022 \times10^{23}$ atoms of oxygen.

My doubt is that how come all these things come together? My teacher just told to memorise. But I can't understand how come we got the Avogadro's constant, and how it's related to gram atomic mass.

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    $\begingroup$ Have you done a quick research of this part of history of chemistry? E.g. starting with history of Mole, atomic mass unit(Dalton) and Avogadro number, following eventual references ? $\endgroup$
    – Poutnik
    Feb 15 '21 at 15:26
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    $\begingroup$ Note that you shall not mix quantities and units as in your wrong equation "Mole = weight/molar mass". Furthermore, descriptive terms or names of quantities shall not be arranged in the form of an equation. $\endgroup$
    – Loong
    Feb 16 '21 at 18:58
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    $\begingroup$ Also note that the concept of "gram atomic mass" is no longer used. $\endgroup$
    – Loong
    Feb 16 '21 at 18:59
  • $\begingroup$ A mole is simply a number. There is no magic in equations involving it. It was originally defined as the number of atoms in 16g of oxygen (later revised to 12g of carbon and recently defined as an exact number). The original definitions were designed to make it easy to compare the mass and atomic counts with the techniques available. $\endgroup$
    – matt_black
    Feb 17 '21 at 1:09
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If I understand your question, you would like to know how the Avogadro number is measured. There are no simple and immediate answer.

One of the possibility is using X-rays to measure with precision the distance $d$ between two atoms in a cubic crystal like $\ce{Si}$, where the atoms $\ce{Si}$ are regularly alined along the three axes Ox, Oy, Oz. As the density of the silicium is known, the volume $V$ of a cube containing $1$ mole ($\pu{28.09 g}$ $\ce{Si}$) may be calculated. Its edge has a length $a = V^{1/3}$ and contains $x$ atoms. As $d$ is known, $x$ may be calculated $x = a/d$. This cube contains ${x^3}$ atoms ; ${x^3}$ is the Avogadro number.

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