What Is a Mole And Why Are Chemists Interested in It?

This activity is designed to introduce a convenient unit used by chemists and to illustrate uses of the unit.

Part I:  What Is a Mole And Why Are Chemists Interested in It?

Counting things is a normal part of everyday life. How many days left until vacation? How many eggs do I need for the recipe? If large numbers of things are involved, we use grouping

strategies to make the numbers easier to manage. For example, 4 more weeks until vacation, tells

us that there are twenty-eight days. One dozen eggs is the common way of expressing the quantity 12. Half of a dozen of anything would be 6 units. One gross is 144 items (12 dozen) and a ream of paper contain 500 sheets.

Chemist are faced with a unique problem when dealing with numbers of atoms or molecules. The particles are so small that any amount of them that we are able to physically handle contains a number of particles so large that there is nothing else in our experience that contains so many units. This incredibly large number calls for a special counting group – the MOLE.

A MOLE is 6.022 x 1023 particles. This is often referred to as Avogadro=s number.  Let=s make sure we understand how big this is. One mole of the element carbon has a mass of 12.01 grams. The smallest particle of an element is an atom. So one mole of carbon contains 602,200,000,000,000,000,000,000 atoms of carbon.

Look for the element carbon on the periodic table. Do you notice anything special about the value 12.01? Explain  

The mass of one mole of the element magnesium is 24.30 grams. How many atoms does a sample of magnesium with a mass of 24.30 grams contain?  

Stated in general terms, the mass of one mole of any element is equal to the

  of that element expressed in grams. The mass of a mole of any element can be found by looking on .

The mass of 0.5000 moles of carbon is and contains

 atoms of carbon.

Remember when dividing numbers written in scientific notation the number portion is divided

normally and the exponents are subtracted. 6.022×1023 divided by 2 is the same as 6.022 x1023/2 x 100.  So the answer is found by dividing 6.022 by 2 = 3.011 and the subtracting exponent 0 from exponent 23. The answer in scientific notation is 3.011×1023 atoms of carbon.

Calculator tip: for exponential notation use the EE or EXP key (not 10^)

If you have a bottle containing 8.10 grams of magnesium, how many Mg atoms are present in the bottle? Show your work. What is different about this problem compared to the last one involving carbon?

Remember that some elements, when alone, exist in the form of diatomic molecules: H2, O2, N2, I2, F2, Cl2, Br2,  Their smallest piece is a molecule containing two atoms. If one mole of oxygen were required for an experiment you would be using O2 the gas.  One mole of O2 would have a mass of    

and contain particles (molecules).

The characteristic unit of the compound CO2 is a molecule. Each CO2 molecule has atoms.  In order to find the mass of one molecule of CO2, it is necessary to add together the atomic masses of each of the atoms in the compound. Atomic masses are expressed in atomic mass units (a.m.u.)

C = 12.01 a.m.u. per atom x 1 atom  = 12.01 a.m.u. O = 16.00 a.m.u. per atom x 2 atoms =  +  32.00 a.m.u.

44.01 a.m.u.

44.01 a.m.u x  6.022 x 1023 molecules CO2

x  1.66 x   10-24 g

= 44.00g CO2 (close!)

1 molecule CO2 1 mole CO2

1 a.m.u.

1 mole CO2

The molar mass of any substance is the mass of 6.022 x 1023 units of the substance. Find the molar masses of the following compounds. Show your work.

H2SO4 Al (NO3)3

52 Exploring the Chemical World, PGCC, 2003

If you know that a dozen tennis balls has a mass of 1.00 lbs, how would you find the mass of one tennis ball?

Using the same mathematical approach, what is the mass in grams of one magnesium atom? Show all work. What is the mass in grams of one molecule of H2SO4?

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