Elements
Atoms
In laboratory practice, you work with much larger quantities of the elements than single atoms or single molecules. A convenient standard quantity is the
mole, the amount of the substance equal in grams to the sum of the atomic weights. (The word mole is a contraction of molecular weight.) One mole of carbon dioxide, therefore, weighs 44.01 grams:
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The mole is a convenient unit for expressing the relative amounts of substances in chemical reactions. The burning of carbon in oxygen can be written with the 2 oxygen atoms bonded in a single O2 molecule:
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The mole is the most common unit used to express the quantity of a chemical substance. For all solids, liquids, and gases, you can convert weight to moles (or moles to weight). For gases, you should memorize the following conversion of volume to moles (or moles to volume): at 0° C and 1 atmosphere pressure, one mole of any gas occupies approximately 22.4 liters. Therefore, the preceding reaction describing the oxidation of carbon means that 12 grams of carbon burned in 22.4 liters of O2 yields 22.4 liters of carbon dioxide.
Many substances do not exist as molecules. For example, the atoms in most inorganic solids are in a three-dimensional structure in which each atom is surrounded by a number of other atoms. In crystals of sodium chloride, no distinct Na and Cl pair can be called a molecule, because each sodium is surrounded by 6 chlorine and each chlorine is surrounded by 6 sodium.
For nonmolecular substances like sodium chloride, the use of the word mole, with its connotation of molecules, is inappropriate. A comparable unit, the
gram formula weight, is used; it is defined as the sum in grams of the atomic weights of all the atoms in the chemical formula of the substance. For sodium chloride (NaCl), one gram formula weight is calculated as
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The mole unit and the gram formula unit are employed in similar calculations. For this reason, many chemists use the term mole to describe quantities of both molecular and nonmolecular substances. Try answering the following problems:
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Problem 1: How many moles of bromobenzene are in 1 kilogram of bromobenzene, C6H5Br?
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Problem 2: What is the mass in grams of neon gas that occupies a volume of 5 liters at 0° C and 1 atmosphere of pressure?
