http://www.ck12.org Chapter 10. The Mole
and trade dealings. Many societies had harsh penalties for using false weights. Today we have state and federal
agencies that make sure the scales used for business are accurate.
Shopping in a grocery store provides many opportunities for purchases by weight. Most fresh meat and fresh
vegetables are marketed by weight, as are flour, sugar, and many other packaged goods.
Determining by Volume
Volume is another way of measuring material. Gases cannot be easily weighed, but their volume can be determined
quickly and accurately. Liquid chemicals as well as foods and beverages are also sold by volume. Gasoline for our
cars and trucks is sold by the gallon. At the grocery store, we can buy a gallon of milk or a two-liter bottle of soda
pop.
Determining by Counting
The third way to indicate how much of something is present is to count the objects. At the grocery store, we could
buy a dozen eggs, six donuts, or two loaves of bread. We might buy movie tickets or t-shirts, based on price per item.
The weight or volume for many of items could be determined, but counting is generally faster for small numbers of
objects and does not require any additional measurement tools.
Converting between Different Measurements
Sometimes one method of measurement is easiest but does not provide us with the information that we need. For
example, say you had a huge sack of pennies and wanted to know how much it was worth. You could count them, but
it might be much faster to weigh the sack. Say you find out that you have 4.801 pounds (2177.5 grams) of pennies. If
you know that each penny weighs 2.5 grams (on average), then you could divide 2177.5 by 2.5, and you would know
that you have about 871 pennies, or $8.71. Such conversions become even more important when dealing with much
more miniscule objects, such as atoms or molecules. Because the numbers involved when talking about amounts of
atoms are so large, a new unit was developed to make discussing such amounts easier. This unit is referred to as the
mole.
The Mole
As we will see in the chapter on gases, the Italian scientist Amadeo Avogadro (1776-1856) proposed that the volume
of a gas is directly proportional to the number of atoms or molecules in a given sample. Due to the vast quantities
of atoms and molecules in an easily observable sample of any material, chemists needed a name that can stand for a
very large number of items. Amole(abbreviated "mol") is the name given to a group of 6.022× 1023 items. This
value is namedAvogadro’s numberas a tribute to his influential work.
The word "mole" is analogous to the word "dozen", in that it is a name that stands for a number. A dozen donuts
is simply another way of saying 12 donuts. Similarly, a mole of donuts would be another way of saying 6.022×
1023 donuts. Obviously, this would be an excessive amount of donuts. However, it becomes a more useful quantity
when talking about objects like molecules. One mole of water molecules is equivalent to 6.022× 1023 molecules of
water. This amount of liquid water has a volume of a little over one tablespoon, which is a very normal amount to
be dealing with.
Technically speaking, the mole is defined as the amount of carbon atoms in exactly 12 grams of carbon-12 (the
isotope of carbon that contains 6 protons and 6 neutrons). This value cannot be determined by literally counting
atoms, but it has been calculated in a variety of ways. When written out to 8 significant figures, the currently