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If the atomic mass of a substance = M  ie., the number of protons and neutrons in an atom of the substance is equal to M  then,

 in M grams of the substance there are Avogadro number of atoms. ie., 6.023 * 10²³  number of atoms.

It is physically not possible to count the number of atoms and molecules.  It is possible to establish a relationship between the mass and number of atoms or molecules.

Molecular mass of oxygen is 32... so in 32 grams of oxygen gas - regardless of volume, temperature, pressure density etc..  there are  6.023 * 10²³  number of molecules.

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