The Avogadro constant
The number of atoms in 12g of carbon-12.
A helium atom is 4 times heavier than a hydrogen atom. A lithium atom is 7 times heavier than a hydrogen atom. In order to get the same number of atoms in a sample of helium or lithium, as the number of atoms in 1 gram of hydrogen, you must take 4grams of helium or 7 grams of lithium.
This same logic applies to molecules.
Water (H2O) has as relative molecular mass of 18. So one molecule of water is 18 times heavier than one atom of hydrogen. Therefore, 18g of water contains the same number of molecules are there are atoms in 1g of hydrogen.
Avogadro constant is written as: 6.022 * 10^23
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