The Avogadro constant was chosen so that the mass of one mole of a chemical compound, in grams, is numerically equal (for all practical purposes) to the average mass of one molecule of the compound, in atomic mass units.
Thus, for example, the mean mass of one molecule of water is about 18.015 atomic mass units; and one mole of water is 6.023×1023 molecules, whose total mass is about 18.015 grams.
The mole is widely used in chemistry as a convenient way to express amounts of reactants and products of chemical reactions. For example, the chemical equation 2H2 + O2 → 2H2O can be interpreted to mean that 2 mol dihydrogen (H2) and 1 mol dioxygen (O2) react to form 2 mol water (H2O).
The mole may also be used to represent the number of atoms, ions, or other entities in a given sample of a substance. The concentration of a solution is commonly expressed by its molarity, defined as the amount of dissolved substance per unit volume of solution, for which the unit typically used is moles per litre (mol/l), commonly abbreviated M
The students must have a sound knowledge of atoms, molecules and compounds. They know how to balance equations. They are used to the concept of relative masses; relative atomic mass, relative molecular mass, relative isotopic mass and relative formula mass.
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