Formula Mass vs.
Gram Formula Mass
AMUs vs Grams
Aim: How can we find the formula mass
and gram formula mass of a compound?
Essential Questions:
1. How can we find the
formula mass and gram
formula mass of a
compound?
Key Words: mole, Atomic
Mass Unit (AMU), formula
mass, gram formula mass,
empirical formula,
molecular formula
2. What is the difference between empirical and molecular
formulas.
2. What is the purpose of finding the gram formula mass
of a compound? Explain.
Smallest Unit of a Compound?
If I have a beaker of water, what is the smallest
unit of that substance?
H2O
The formula of water is the smallest unit of
water.
Formula Mass
If the formula is the smallest unit of a
substance, how do we calculate the
formula mass of a substance?
We take the sum of the atomic masses of that substance.
Let’s find the formula mass for:
Al: _____
O2: _____
Sodium Sulfate: _____
How do we find the atomic mass of
Sodium Sulfate?
Step 1 : Write the correct formula!
Na2SO4
Element
Atomic Mass
# of Atoms
(Subscript)
Total
x
=
x
=
x
=
Formula Mass:
Always Expressed in AMU!
Picture This!
You are a chemical technician.
A chemist asks you for 1 AMU of Na2SO4
How will you give it to him? What will you do?
Convert to Grams
Gram Formula Mass!
Find the Gram Formula Mass for
Glucose
Element
Atomic Mass
# of Atoms
(Subscript)
Total
x
=
x
=
x
=
Find the Gram Formula Mass of
Zirconium Phosphate
Element
Atomic Mass
# of Atoms
(Subscript)
Total
x
=
x
=
x
=
HW
Find the gram formula mass for the sheet from
(1) KMuO4 to (11) H2CO3
(Notes)
I. Formula Mass
A. The Formula of a substance is the smallest
unit of that substance.
B. Calculating the Formula Mass of a substance
1. Step 1: Find the correct formula
2. Step 2: Find the gram atomic masses of each
element (see periodic table and round off number
to the nearest tenth).
3. Step 3: Multiply each atomic mass by the subscript
in the formula.
4. Step 4: Find the sum of the masses.
(Notes)
A. Use the following chart to avoid errors
B. The formula mass is always expressed in
AMU’s
(Notes)
II. Gram Formula Mass
A. Convert formula mass to grams and you will have
the gram formula mass.