Unit 8: The
Mole
Formula weight vs.
Molecular weight
 Formula
weight – sum of the atomic
weights of each atom in its chemical
formula

The formula weight of sulfuric acid (H2SO4) is
98.1 amu
 If
the chemical formula is that of a
molecule, then the formula weight is also
called the molecular weight

The molecular weight of glucose (C6H12O6)
is 180.0 amu
Percentage Composition from
Formulas
 To
find the percentage of each element
that is in a compound, the following
equation is used:
% element = (number of atoms of that element)(atomic weight of element)
formula weight of compound
x 100
The Mole
(the real mole, not the chemistry one
The Mole



The term, mole, is used to express the number
of atoms, ions, or molecules (mol)
A mole is the amount of matter that contains
as many objects (atoms, molecules, or
whatever objects we are considering) as the
number of atoms in exactly 12 g of 12C
From experiments, scientists have determined
that number to be 6.0221421 x 1023
Avogadro’s Number
 The
number, 6.022 x 1023 is called
Avogadro’s number
 This number too large to imagine
 Spreading 6.02 x 1023 marbles over the
entire surface of the Earth would produce
a layer about 3 miles thick
 If you lay 6.02 x 1023 pennies side by side in
a straight line, they would encircle the
Earth 300 trillion (3 x 1014) times
Molar Mass
 What

is molar mass?
Molar mass is the amount of the mass (in
grams) of 1 mol of a certain element
 Cl
has a molar mass of 35.5 g/mol
 NaCl has a molar mass of 58.5 g/mol
 Molar mass is always numerically equal to
the formula weight
22.4 L
1 mol
Liters of a gas
1 mol
22.4 L
6.02 x 1023 particle
1 mol
Mole
MM
1 mol
particle
1 mol
6.02 x 1023 particle
1 mol
MM
Mass (grams)
Empirical Formula
 Shows
the relative number of atoms of
each element in a formula
 Steps to determine empirical formula:
1.
2.
3.
If given % of element, change the % to g
Determine the moles of each element
(divide by the MM)
Divide by the lowest mole value
Empirical Formula Problem
 Ascorbic
acid (vitamin C) contains 40.92%
C, 4.58% H, and 54.50% O by mass. What
is the empirical formula of ascorbic acid?
Molecular Formula
 Molecular
formula can be obtained only if
you are given the molecular weight
 The subscripts in the molecular formula of
a substance are always a whole number
multiple of the corresponding subscripts in
its empirical formula
Whole-number multiple =
molecular weight
empirical formula weight
Molecular Formula Problem
 What
is the molecular formula of ethylene
glycol, substance used in automobile
antifreeze, which is composed of 38.7% C,
9.7%H, and 51.6% O by mass?