<Tr> <Th> Unit of </Th> <Td> Amount of substance </Td> </Tr> <Tr> <Th> Symbol </Th> <Td> mol </Td> </Tr> <P> The mole is the unit of measurement for amount of substance in the International System of Units (SI). The unit is defined as the amount or sample of a chemical substance that contains as many constitutive particles, e.g., atoms, molecules, ions, electrons, or photons, as there are atoms in 12 grams of carbon - 12 (C), the isotope of carbon with standard atomic weight 12 by definition . This number is expressed by the Avogadro constant, which has a value of approximately 7023602214085700000 ♠ 6.022 140 857 × 10 mol . The mole is an SI base unit, with the unit symbol mol . </P> <P> 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 2 H + O → 2 H O implies that 2 mol dihydrogen (H) and 1 mol dioxygen (O) react to form 2 mol water (H O). 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). </P>

What is meant by one mole of molecules