2. Avogadro’s Number and the Mole
Atoms, ions, and molecules are very small, so even tiny samples have a huge
number of particles. To make counting such large numbers easier, scientists
use the same approach to represent the number of ions or molecules in a
sample as they use for atoms. The SI unit for amount is called the mole
(mol). A mole is the number of atoms in exactly 12 grams of carbon-12.
The number of particles in a mole is called Avogadro’s number or
Avogadro’s constant. One way to determine this number is to count the
number of particles in a small sample and then use mass or particle size
to find the amount in a larger sample. This method works only if all of the
atoms in the sample are identical. Thus, scientists measure Avogadro’s
number using a sample that has atoms of only one isotope.
The most recent measurement of Avogadro’s number shows that it is
× 1023 units/mole. In this class, the measurement is rounded
to × 1023 units/mol. Avogadro’s number is used to count any kind of
particle.

3. The Mole Is a Counting Unit
Moles
Multiply by x 1023
Divide by x 1023
# of Particles

4. 1. How many ions are there in 0.187 mol of Na+ ions?
2. How many atoms are there in 1.45 × 10−17 mol of arsenic?
3. How many molecules are there in mol of acetic acid, C2H4O2?
4. How many formula units are there in 5.9 mol of NaOH?
1.13 x 1023 ions of Na+
8.73 x 106 ions of arsenic or 8,730,000 ions of arsenic
2.544 x 1024 ions of C2H4O2
3.6 x 1024 ions of NaOH

5. 1. How many moles of xenon do 5.66 × 1023 atoms equal?
2. How many moles of silver nitrate do × 1015 formula units equal?
3. A biologist estimates that there are 2.7 × 1017 termites on Earth. How many moles of termites is this?
4. How many moles do 5.66 × 1025 lithium ions, Li+, equal?
0.940 moles of xenon
4.796 x 10-9 moles of silver nitrate
4.5 x 10-7 moles of termites
9.40 x 101 moles of Li+ or 94.0 moles of Li+