Untitled - Kelly Walsh High School

34 CHEMISTRY FOR THE UTTERLY CONFUSED
We can balance most simple reactions by this trial and error method, by inspection,
but some reactions, redox reactions, often require a different system of
rules. We will show you how to balance these redox reactions in Chapter 18.
3-2 Avogadro’s Number and Molar Mass
Quick Tip
The mole (mol) is the amount of a substance that contains the same number of
particles as atoms in exactly 12 grams of carbon-12. This number of particles
(atoms, molecules, or ions) per mole is Avogadro’s number and is numerically
equal to 6.022 10 23 particles. The mole is simply a term that represents a certain
number of particles, like a dozen or a pair. The mole also represents a certain
mass of a chemical substance.
The substance’s molar mass is the mass in grams of the substance that contains
one mole of that substance. In the previous chapter, we described the atomic mass
of an element in terms of atomic mass units (amu). This was the mass associated
with an individual atom. At the microscopic level, we can calculate the mass of a
compound by simply adding together the masses in amu’s of the individual elements
in the compound. However, at the macroscopic level, we use the unit of
grams to represent the quantity of a mole.
6.022 10 23 particles 1 mol molar mass in grams
This relationship gives a way of converting from grams to moles to particles and
vice versa. If you have any one of the three quantities, you can calculate the
other two. For example, the molar mass of iron(III) oxide, Fe 2O 3 (rust), is
159.689 g/mol [(2 55.846 g/mol for Fe) (3 15.999 g/mol for O)].
Therefore, if we had 50.00 g of iron(III) oxide, we could calculate both the
number of moles and the number of particles present.
(50.00 g Fe 2 O 3 )a 1 mol Fe 2 O 3
159.689 g b 0.313108605 mol 0.3131 mol Fe 2 O 3
(0.313108605 mol Fe2O3 )a 6.022 1023 particles
b
1 mol
1.886 10 23 Fe 2 O 3 particles