Molar Quantities
A quantity of particles, atoms, or molecules of any substance that is equal to Avogadro's number is called one mole of that substance. Avogadro's number, a constant, is an immense number equal to 602,000,000,000,000,000,000,000, or 6.02x1023 particles of any material. Therefore, one mole of a substance is equal to 6.02x 1023 particles. Molar quantities, then, are amounts of chemical substances expressed as moles. For example, one mole of salt is Avogadro's number of salt molecules, two moles of hydrogen ions equals two times Avogadro's number of protons.
Molar quantities, by definition have known numbers of particles. Such quantities, however, may have very different weights. For example, one mole of water and one mole of carbon dioxide each have the same number of molecules, but the mole of water weighs 18 g, while the mole of carbon dioxide weighs about 48 g. The difference in mass occurs because one mole of any substance is equal to its gram-molecular weight. Gram-molecular weight is the molecular weight of a molecule expressed as grams. The molecular weight is, in turn, the sum of the atomic weights of the constituent atoms within the molecule. For example, the gram molecular weight of water, two hydrogen atoms bonded to one oxygen atom, is 18 grams (hydrogen has anatomic weight of about 1, and oxygen has an atomic weight of 16). Therefore, one mole of water contains 6.02x1023 molecules and weighs 18 g.
Molar quantities are used to express concentrations of substances dissolved in solution. Molarity is a molar quantity of concentration equal to the number of moles of a substance that are dissolved in 1 liter of liquid. Molarity, expressed as moles per liter, takes into account the total number of particles in solution rather than expressing concentration simply as a percentage. For example, a molar salt solution of 2.0 (mol/liter) is made by dissolving 2 moles of salt into 1 liter of water. In this solution, the exact number of salt molecules is known because it is a molar quantity. Also, the exact weight of salt needed is also known since two moles of salt equal two gram-molecular weights of sodium chloride, or about 116 g of salt.
Some molar quantities have exact volumes, called molar volumes, as well. For example, one mole of any gas always occupies approximately 22.4 liters of space at standard temperature and pressure. Also, one mole of liquid water always occupies 18 ml (cm[sup3 ]) of space at standard temperature and pressure.
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