This is why we state the atomic and molecular masses in units of grams per mole or g/mol. Lithium for instance has an atomic mass of 6.941 grams and this is equal to one mole of lithium. Using carbon as a reference, the atomic weights you see in the periodic table are also equal to one mole of those substances: This means that the atomic mass or atomic weight (12 grams) of carbon is equal to exactly 1 mole of carbon. Why use 12 grams? This is the theoretical atomic mass of the Carbon-12 isotope (6 protons and 6 neutrons). The value given 6.023 x 10 23 is called Avagadro's number for the scientist that found the number of atoms in 12 grams of carbon 12. We use the mole (mol) to represent the amount of substances in chemistry because the numbers of atoms and molecules in each substance is so large. So if you had a mole of donuts you would have 6.023 x 10 23 donuts and a serious stomach ache. So the mole is the title used for the amount 6.023 x 10 23 much the same way the word "dozen" is used for the amount 12. The MOLE (mol) is a unit of measurement that is the amount of a pure substance containing the same number of chemical units (atoms, molecules etc.) as there are atoms in exactly 12 grams of carbon-12 (i.e., 6.023 X 10 23). So the question you are probably asking is "what is a mole?" and "why do we need to use it?" A chemist would say that 2 moles of hydrogen react with 1 mole of oxygen to form 2 moles of water. While this is not completely incorrect it is not the way a chemist would state the equation. And we introduced balancing reactions in terms of keeping the number of atoms the same on each side of the reaction equation. We would say 2 atoms of hydrogen react with 1 atom of oxygen to form 2 water molecules. Previously as we have discussed reactions, we have discussed the changes in terms of either molecules or atoms: Interconverting Masses, Moles and Numbers of Particles - Chemistry Tutorial
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