Download Percent Composition, Empirical and Molecular Formulas Chemists

February 26, 2014
Percent Composition, Empirical and Molecular
Formulas
Chemists create new compounds for
industry, pharmaceutical, and home use
Analytical chemist analyzes new compound
to provide proof of composition and
chemical formula.
Ex: Aspirin- C9H8O4
February 26, 2014
Percent Composition: the percent by mass of each
element in a compound
mass of element
mass of compound
x 100 = percent by mass
*If given mass of each element--just divide.
Example: A compound was found to contain
49.98g C and 10.47g H. What is the percent
composition?
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Given the chemical formula, to find the percent
composition...
1. Assume you have 1 mole of the compound
2. Find the mass of each element (molar mass x
subscript)
3. Use formula for percent by mass
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Example 1: Find the percent composition of H2O
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Example 2: Find the percent composition of sodium
hydrogen carbonate (NaHCO3, or baking soda)
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For a new compound, if the elements in a sample
are known and the percent composition is known,
we can find the formula of the compound.
1. Find empirical formula
2. Find molecular formula
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Empirical Formula: Smallest whole number ratio of
moles of elements in the compound
• determines subscripts
• may or may not be the same as molecular formula
For example:
The empirical formula of hydrogen peroxide is HO
(H1O1)
The molecular formula of hydrogen peroxide is H2O2
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How to find empirical formula...
If you are given the percent composition in a given
mass of compound...
1. Assume you have 100g of the compound
> Percent by mass = mass of element
2. Convert each mass to moles
3. If the moles are not whole numbers, divide the
moles by the smallest mole of an element
4. If still not whole numbers, multiply by smallest
number to produce a ratio of whole numbers
*If you are given the mass of each element, start at
step 2.
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Example 3: Percent composition of an oxide of
sulfur is 40.05% S and 59.95% O
1.
Assume you have 100g of
the compound
> Percent by mass = mass
of element
2. Convert each mass to
moles
3. If the moles are not whole
numbers, divide the moles
by the smallest mole of an
element
4. If still not whole numbers,
multiply by smallest
number to produce a ratio
of whole numbers
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Example 4: Determine the empirical formula of
methyl acetate which has the collowing percent
composition: 48.64% C, 8.16% H, and 43.20% O
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Molecular Formula: specified actual number of each
element in one molecule
*Different substances (different molecular formulas)
can have the same empirical formula.
Ex: Acetylene and Benzene both have the
empirical formula CH
Acetylene: C2H2
Benzene: C6H6
http://www.citycollegiate.com/benzene3.htm
February 26, 2014
To determine the molecular formula for a compound...
1. Take the experimentally determined molar mass
(will be given to you)
2. Divide by mass of empirical formula
***This will tell you how many times bigger the molar
mass is***
3. Multiply empirical formula by result.
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Example 5: What is the molecular formula of succinic
acid? The molar mass of succinic acid is 118.1g/mol.
The empirical formula is C2H3O2
1. Take the
experimentally
determined molar
mass (will be given to
you)
2. Divide by mass of
empirical formula
***This will tell you
how many times
bigger the molar mass
is***
3. Multiply empirical
formula by result.
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Example 6: A compound was found to contain 49.98g
C and 10.47g H. The molar mass is 58.12g/mol.
Determine the molecular formula.
1.
2.
Determine empirical
formula
Determine
molecular formula
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Example 7: A colorless liquid composed of 46.68% N
and 53.32% O has a molar mass of 60.01g/mol.
What is the molecular formula?
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Hydrate: A compound that has a specific number of
water molecules bound to its atoms.
Example: Opal (a gemstone) is composed of Silicon
dioxide (
). The coloring is the result of water
in the mineral.
http://en.wikipedia.org/wiki/File:8ct_Lightning_Ridge_Black_Opal.JPG
http://en.wikipedia.org/wiki/File:10_7cts_Brazilian_Crystal_Opal.jpg
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Hydrate: A compound that has a specific number of
water molecules bound to its atoms.
Example: Desiccants--anhydrous (w/o water) form of
hydrate can absorb water from air
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Hydrates
• A formula for a hydrate indicates the # of water
molecules associated with the formula of the
compound
> Example: Na2CO3 10H2O
(Sodium carbonate decahydrate)
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To determine the formula for a hydrate...
1. Find moles of water associated with 1 mole of
hydrate
> Mass hydrate compound
> Heat sample of hydrate to drive off water
> Mass anhydrous (w/o water) compound
> Find difference:MassH2O=Masshydrate - Massanhydrous
2. Find moles of water
3. Find moles of anhydrous compound
4. Calculate ratio of moles of water to compound
moles water
moles anhydrous compound
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Example 8: A mass of 2.50g of hydrated copper
sulfate (CuSO4 xH2O) is heated. After heating, 1.59g
anhydrous copper sulfate (CuSO4) remains. what is
the formula of the hydrate?
1. Find moles of water associated with 1
mole of hydrate
> Mass hydrate compound
> Heat sample of hydrate to drive
off water
> Mass anhydrous (w/o water)
compound
> Find difference:MassH2O=Masshydrate
- Massanhydrous
2. Find moles of water
3. Find moles of anhydrous compound
4. Calculate ratio of moles of water to
compound
moles water
moles anhydrous compound
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Example 9: A mass of 11.75g of hydrated cobalt (II)
chloride is heated. After heating, 9.25g anhydrous
cobalt (II) chloride remains. what is the formula of the
hydrate?
1. Find moles of water associated with 1
mole of hydrate
> Mass hydrate compound
> Heat sample of hydrate to drive
off water
> Mass anhydrous (w/o water)
compound
> Find difference:MassH2O=Masshydrate
- Massanhydrous
2. Find moles of water
3. Find moles of anhydrous compound
4. Calculate ratio of moles of water to
compound
moles water
moles anhydrous compound
February 26, 2014
Example 10: A hydrate is found to have the following
percent composition: 48.8% MgSO4 and 51.2% H2O.
What is the formula and name for this hydrate?
*Hint: Think about what percent composition means!
Look back on how to find empirical formula from
percent composition
February 26, 2014