Chemistry Reference
In-Depth Information
A. Calculate the percent composition of each of (a) methyl alcohol,
and (b) TNT,
CH
4
O,
C
7
H
5
N
3
O
6
.
7.3
The Mole
The atomic mass unit (amu) is an extremely small unit, suitable for measuring
masses of individual atoms and molecules. However, to measure masses on lab-
oratory balances takes a huge number of atoms, molecules, or formula units.
Chemists have to weigh a large collection of formula units, so that the total
mass is measurable on a laboratory balance. (If we try to weigh one grain of
rice on a scale designed to weigh people, we will get an inkling of the prob-
lem of measuring the mass of one atom or molecule. See Figure 3.7.)
The
mole
(abbreviated mol) is the standard chemical unit used to measure
the quantity of a substance. A mole is defined as the number of atoms in
exactly
12 g of The mole is equal to particles. Thus the
mole is a number, essentially like a dozen, except very much larger. This num-
ber is about the number of grains of sand that would fit into a sphere the size
of the Earth. (See Problem 2.154.) This number is known as
Avogadro's number.
Remember the value of this number to at least three significant digits.
Avogadro's number was set at so that the atomic mass of
each element and the number of grams per mole of that element have the same
numeric value, although in different units. The atomic mass of is 12.00 amu,
and 12.00 g is the mass of 1.000 mol of The formula mass of any com-
pound or element is also equal to its number of grams per mole. The formula
mass of a substance in units of grams per mole is called the
molar mass
of the
substance. (Molar mass can be abbreviated MM.)
The mole is the basic unit of
quantity of chemical substance.
12
C
12
C.
10
23
6.0221367
10
23
6.0221367
12
C
12
C.
The molar mass of any substance
is equal to the number of grams
per mole of that substance.
ENRICHMENT
It is also possible to think of a mole as the
number of atomic mass units in 1 gram.
The number of atomic mass units per gram is equal
to Avogadro's number.
Practice Problem 7.5
Calculate the num-
ber of inches in 1 foot, knowing that a certain shoe
box is 4.00 in. tall and that a stack of 1 dozen of these
boxes is 4.00 ft tall.
EXAMPLE 7.5
Use the value of Avogadro's number, the mass of
one atom (exactly 12 amu), and the definition
of a mole to calculate the number of atomic mass
units per gram.
12
C
Solution
10
23
12
C atoms
1 mol
12
C
a
1 mol
12
C
12.000 g
6.022
a
12.000 amu
1
12
C atom
b
b
Avogadro's number
Mass of
Mass of
1 mol
1 atom
10
23
amu/g
6.022
