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The mole is represented by Avogadro's number, which is 6.02×1023 mol-1.
Define and memorize Avogadro's number
The mole allows scientists to calculate the number of elementary entities (usually atoms or molecules) in a certain mass of a given substance.
Avogadro's number is an absolute number: there are 6.022×1023 elementary entities in 1 mole. This can also be written as 6.022×1023 mol-1.
The number of molecules in a mole is defined so that the mass of one mole of a substance, expressed in grams, is equal to the substance's mean molecular weight. For example, hydrogen has an atomic weight of 1.0079 amu or 1.0079 g/mol. By this definition, 1 mol of hydrogen weighs 1.0079 g.
The chemical changes observed in any reaction involve the rearrangement of billions of atoms. It is impractical to try to count or visualize all these atoms, but scientists need some way to refer to the entire quantity. They also need a way to compare these numbers and relate them to the weights of the substances, which they can measure and observe. The solution is the concept of the mole, which is very important in quantitative chemistry.
Amadeo Avogadro first proposed that the volume of a gas at a given pressure and temperature is proportional to the number of atoms or molecules, regardless of the type of gas. Although he did not determine the exact proportion, he is credited for the idea.
Avogadro's number is a proportion that relates molar mass on an atomic scale to physical mass on a human scale. Avogadro's number is defined as the number of elementary particles (molecules, atoms, compounds, etc.) per mole of a substance. It is equal to 6.022×1023 mol-1 and is expressed as the symbol NA.
Avogadro's number is a similar concept to that of a dozen or a gross. A dozen molecules is 12 molecules. A gross of molecules is 144 molecules. Avogadro's number is 6.022×1023 molecules. With Avogadro's number, scientists can discuss and compare the large numbers of tiny atoms and molecules that we encounter in chemistry.
The mole (abbreviated mol) is the SI measure of quantity of a "chemical entity," such as atoms, electrons, or protons. It is defined as the amount of a substance that contains as many elementary entities as there are atoms in 12 grams of pure carbon-12. So, 1 mol contains 6.022×1023 elementary entities of the substance.
Chemical Computations with Avogadro's Number and the Mole
Avogadro's number is fundamental to understanding both the makeup of molecules and their interactions and combinations. For example, since one atom of oxygen will combine with two atoms of hydrogen to create one molecule of water (H2O), one mole of oxygen (6.022×1023 of O atoms) will combine with two moles of hydrogen (2 × 6.022×1023 of H atoms) to make one mole of H2O.
Another property of Avogadro's number is that the mass of one mole of a substance is equal to that substance's molecular weight. For example, the mean molecular weight of water is 18.015 atomic mass units (amu), so one mole of water weight 18.015 grams. This property simplifies many chemical computations.
If you have 1.25 grams of a molecule with molecular weight of 134.1 g/mol, how many moles of that molecule do you have?
The amount of a substance that weighs the same as 12 grams of carbon-12, The amount of a substance that occupies the same volume as 12 grams of carbon-12, The amount of a substance containing as many elementary entities as atoms in 12 grams of carbon-12, or The amont of a substance that contains the same number of protons as 12 grams of carbon-12
Source: Boundless. “Avogadro's Number and the Mole.” Boundless Chemistry. Boundless, 26 May. 2016. Retrieved 28 May. 2016 from https://www.boundless.com/chemistry/textbooks/boundless-chemistry-textbook/mass-relationships-and-chemical-equations-3/molar-mass-41/avogadro-s-number-and-the-mole-220-3701/