Learning Objectives

Moles

Up to this point, we have thought of the numbers that we placed in front of the compounds in the chemical equation as the number of actual molecules. This is true in a manner of speaking but molecules are so small that it is not practical to state the numbers of molecules as such.

Rather, we use the term mole to describe a given quantity of molecules or atoms. The coefficients in a reaction equation represent moles. Definitions of mole include the following…

1. Avogadro's Number is the number of molecules that are present in one mole and equals 6.023 x 10²³ molecules.

This number is incredibly large.  We are not really able to really comprehend a number of this magnitude.

2. One mole of any gas will occupy 22.4 liters at standard temperature and pressure. We define standard temperature as zero degrees centigrade and standard pressure as one atmosphere...sea level.

3. One mole = molar mass of a compound in grams. This is the single most important definition of a mole.

The molecular formula of a compound also represents the number of moles of a given element within the formula. One mole of CH₄ has one mole of carbon atoms and four moles of hydrogen atoms.

Molar Mass

The molar mass (molecular weight) of a compound is the number of grams equal to the combined atomic masses of all the elements in the compound.  The formula mass equals the combined atomic masses in amu units. The mass of 1 carbon atom equals 12.0107 amu.  Thus the mass of one mole of carbon atoms equals 12.0107 g.

Let's calculate the molar mass of water.

Now let's try sulfuric acid, the acid found in your car battery...H₂SO₄.

Moles as a Conversion Factor

Since the molar mass equals one mole, this definition can be used as a conversion factor. This conversion factor will either be (molar mass in grams) divided by (1 mole) or (1 mole) divided by (molar mass in g). This is a very important conversion in chemistry.

How many moles are in 8 g of methane CH₄?

How many grams of NaOH are needed for 0.25 mole?

Now let's try a little harder problem. How many molecules are present in 66 g of CO₂?

Problems

1. Calculate the molar mass of the following compounds.

Na₂SO₄  answer

CaCO₃  answer

2. How many moles are in 100 g of CO₂?

3. How many grams will be present in 2 moles of  Na₂CO₃?

4. How many moles of water are in a 20,000 gal pool?  Water has a density of 1 g/mL.  There are 4 quarts in a gallon and 1.06 quarts in one liter.

5. How many atoms are present in 66 g of CO₂?

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Copyright © January 2001 by Richard C. Banks...all rights reserved.