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The molecular mass is the same numeric
value as the atomic mass but the unit is
different
Atomic mass is measured in atomic mass
units (amu) and molecular mass is measured
in grams
So molecular mass is the atomic mass with
the unit “grams” or “g”
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Chlorine has an atomic mass of 35.453 amu
Therefore, its molar mass is… 35.453 grams
The number is the same, just change the unit
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Formula weight of a compound is the sum
of the atomic masses of the atoms as given
in the formula of the compound.
◦ Sum of molecular masses
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Steps:
1. Determine how many moles of each element
are in the compound using the subscripts.
2. Find the molecular mass of each element in the
compound using #1 and atomic mass.
3. Add it all together
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We will use a table that looks like this…
Element
# Moles
Atomic
Mass
Molecular
Mass
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1. Determine moles first…
◦ Subscripts
 No parentheses- moles = subscripts
 parenthesizes– moles = outer subscript x inner subscript
Element
# Moles
Aluminum
1
Bromine
3
Atomic Mass
Molecular
Mass
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2. To find the molecular mass of aluminum and bromine:
Element

# Moles
Atomic Mass
Molecular Mass
Aluminum
1
26.98 g
26.98 g
Bromine
3
79.9 g
239.70 g
3. Add elements molar masses together to get compounds
molar mass:
Element
# Moles
Atomic Mass
Molecular Mass
Aluminum
1
26.98 g
26.98 g
Bromine
3
79.9 g
239.70 g
= 266.68 g/mol AlBr3
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Percent composition is the percent that you
find of any ONE element in a compound of
elements.
Percent is found by taking the PART that you
are looking for and dividing it by the WHOLE.
◦ Then multiply by 100 to move decimal.
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What percent of 92 is 42?
◦ In other words, the part “42” of the whole
“92” is what percent?
◦ 42/92 x 100 = 45.7%
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What is the percent composition of calcium in
calcium phosphate?
Element
Ca
P
# Moles
3
2
Molecular Mass
120.24 g
61.9476 g
O
8
127.9952 g
= 310.1828g / mol
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Now, take the PART (Ca) and divide it by the
WHOLE. Multiple by 100.
Element
# Moles
Molecular Mass
Ca
3
120.24 g
P
2
61.9476 g
O
8
127.9952 g
= 310.1828g / mol
120.24 g Ca / 310.1828 g Ca3(PO4)2 x 100 = 38.8% Ca