Using the Mole Concept in Calculations

Avogadro s number is clearly not a useful number for counting ordinary objects. However, when this inconceivably large number is used to count inconceivably small objects, such as atoms and molecules, the result is a quantity of material that is easily within our grasp, essentially a handful.  

Throughout the text, the mole concept will provide conversion factors for problem-solving situations. With each new situation, we will explore how the mole concept applies. For now, we will deal with the relationship between numbers of atoms and the mole. Consider the statement 1 mol S = 6.022 X 1(P S atoms = 32.06 g S. This allows us to write the conversion factors 

In calculations requiring the Avogadro constant, students often ask when to multiply and when to divide by. One answer is always to use the constant in a way that gives the proper cancellation of units. Another answer is to think in terms of the expected result. In calculating a number of atoms, we expect the answer to be a very large number and certainly never smaller than one. The number of moles of atoms, conversely, is generally a number of more modest size and will often be less than one. 

Example 2-8 illustrated The balance was set to zero (tared) when just the weighing boat was present. The sample of sulfur weighs 4.07 g. 

To ensure the maximum precision allowable, carry at least one more significant figure in well-known physical constants than in other measured quantities. 

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