percent composition, empirical and molecular formulas
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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
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Percent Composition: the percent by mass of each element in a compound
x 100 = percent by massmass of element
mass of compound
*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 element2. Convert each mass to moles3. If the moles are not whole numbers, divide the
moles by the smallest mole of an element4. 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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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
Example 3: Percent composition of an oxide of sulfur is 40.05% S and 59.95% O
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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
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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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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.
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
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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. Determine empirical formula
2. 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:10_7cts_Brazilian_Crystal_Opal.jpg
http://en.wikipedia.org/wiki/File:8ct_Lightning_Ridge_Black_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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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
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?
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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
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?
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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
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