10.1. The Mole Concept http://www.ck12.org
FIGURE 10.1
Amadeo Avogadro (1776-1856)accepted value is 6.0221413× 1023. However, because we don’t usually deal with other measurements that are this
precise, the rounded value of 6.022× 1023 is sufficient for any situations that you are likely to encounter.
The mole is such an important parameter in chemistry that Mole Day was established in the U.S. on October 23,
where it is celebrated from 6:02 AM to 6:02 PM. The day was originally established to generate interest in chemistry.
The scale of Avogadro’s number is nearly incomprehensible in terms of everyday objects. This can be illustrated
with examples like the following problem, in which we use the dimensional analysis technique from the chapter
Measurementto convert between units.
Example 10.1
If you were given one million dollars every second, how long would it take to accumulate a mole of dollars?
Answer:
1 mol of dollars×(^6.^022 ×^10
(^23) dollars
1 mol of dollars )(
1 sec
1 , 000 ,000 dollars)(
1 min
60 sec)(
1 hour
60 min)(
1 day
24 hours)(
1 year
365 days) =^1.^91 ×^10
(^10) years
Even at this very fast rate, it would take over 19 billion years to obtain a mole of dollars. To put this into perspective,
the universe is believed to be about 14 billion years old, and our Sun will run out of fuel and burn out about 6 billion
years from now.
Representative Particles
When we talk about one mole of a particular chemical substance, it is important to know exactly how much material
is being referenced. For example, the phrase "a mole of water molecules" is often shortened to "a mole of water."
When we talk about moles of a molecular substance, it is assumed that we are referring to moles of molecules.
Similarly, the phrase "five moles of carbon dioxide" would refer to five moles of CO 2 molecules. In these cases, the
molecule is sometimes called arepresentative particle, which is simply the smallest unit of a substance that retains
its chemical identity.
What about when we are discussing non-molecular substances? Recall that an ionic compound does not exist as
discrete molecules, but rather as an extended three-dimensional network of ions called a crystal lattice. The empirical
formula of an ionic compound tells us the ratio of the ions in the crystal. In these cases, one mole of the substance
is assumed to mean one mole of each ion in the empirical formula. For example, one mole of sodium oxide (Na 2 O)
refers to two moles of sodium ions (Na+) and one mole of oxide ions (O^2 −). The empirical formula gives you the
representative particle for ionic substances. Unlike molecules, these representative particles do not exist as isolated
units (sodium oxide does not exist as small clusters of three atoms each).