Gram Formula Mass or Molar Mass
• sum of the atomic masses of all the atoms in a
compound
Example:C4H6O2
Element
Quantity
Atomic Mass
of Element
Total Mass of
Element in
Compound
C
4
12
48
H
6
1
6
O
2
16
32
Total
86 g/mol
Steps
1. Determine the number of atoms of every
element present in the formula
2. Look up the atomic mass of each element
on the periodic table
3. Multiply step one by step two for each
element
4. Add the products of step three and round
to the appropriate number of significant
digits
Try these…
•
•
•
•
•
•
MgCl2
NaOH
CaCl2
(NH4)2CO3
3H2O
CuSO4 5H2O
AMU vs. gram
• Atomic Mass Unit
• 1.00 amu = 1.66 x 10-24g
• 1/12 of the mass of an atom of carbon-12
Mole
• Unit of measurement
• Think of it as a term
like “dozen”
• 1 mole=
6.02 x 10 23
molecules=
Avogadro’s number
Mole Central Station
Volume
at STP
1 mole = 22.4 L
22.4 L
1 mole
X
Number of
Molecules
X
1 mole
6.02 x 10 23
=
Mass
Moles
6.02 x 10 23
1 mole
X
1 mole = GFM
GFM
1 mole
Sample Question 1:
• How many moles are in 25 grams of NaCl?
Element
Quantity
Atomic
Mass of
Element
Total Mass
of Element
in
Compound
Na
1
23
23
Cl
1
35.5
35.5
Total
25 g x 1 mole =
58.5 g
0.43 moles
58.5
Sample Question 2:
• How many moles are in 5.8 x 10 25
molecules of HCl?
5.8 x 10 25 molecules x 1 mole
=
6.02 x 10 23 molecules
96.3 moles
Sample Question 3:
• What volume does 2.0 moles of MgBr2
occupy?
2.0 moles x 22.4 L =
1 mole
44.8 L
2 step problems
• What volume does 33 grams of H2O
occupy?
Step 1: Convert 33 grams to moles
33 g x 1 mole = 1.83 moles
18 g
Step 2: Convert moles to volume
1.83 moles x 22.4 L = 41.1 L
1 mole
2 Step
• How many molecules are in 45 grams of
H2SO4?
45 g x 1 mole x 6.02 x 10 23 molecules =
98 g
1 mole
2.76 x 10 23 molecules
How many atoms?
2.76 x 10 23 x 7= 1.9 x 1024
Practice
• How many atoms are in 345 g of Fe?
• How many atoms are in 210 g of H2O?
• How many moles are in 36 L of N2?
• How many molecules are in 22.4 L of
HBr?