A mole (symbol Mol) is the base unit of quantity of a substance in the metric system. A mole is the quantity of a substance that contains 6.02 x 1023 units. A "unit" is the smallest measurable entity in the substance, generally either an atom or a molecule. The number of units in a mole was determined by Italian chemist (a scientist specializing in the composition, structure, properties, and reactions of matter) Amedeo Avogadro (1776-1856), one of the founders of modern physical chemistry. For this reason, the value 6.02 x 1023 is called Avogadro's number. One mole of a substance is equal to the substance's atomic weight (the average weight of an atom of an element) or molecular weight, in grams. You often want to know how many molecules you have in a sample of a substance. Counting the molecules individually would be completely impractical. Even if you had a way to see the individual molecules, there are just too many, even in a tiny sample. Moles were defined to solve the problem of counting large numbers of molecules. With moles, you count the number of molecules in the sample by weighing it.
The mole is useful in chemistry because it allows different substances to be measured comparably. Using the same number of moles of two substances, both amounts have the same number of molecules or atoms. The mole makes it easier to understand chemical equations in practical terms. So the equation: 2H2 + O2 → 2H2O
can be understood, as "two moles of hydrogen plus one mole of oxygen yields two moles of water." Moles are useful in chemical calculations because they enable the calculation of yields and other values when dealing with particles of different mass. Number of particles is a more useful unit in chemistry than mass or weight, because reactions take place between atoms (for example, two hydrogen atoms and one oxygen atom make one molecule of water) that have very different weights (one oxygen atom weighs almost 16 times as much as a hydrogen...
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