Empirical and Molecular Formulas Notes.notebook
January 23, 2014
Empirical and Molecular Formulas
• Empirical Formula: the simplest formula for a compound
• Molecular Formula: the same as or a multiple of the empirical formula, and is based on the actual number of atoms of each type in the compound
Examples:
Empirical
Molecular
H2O2
CH3
C6H12O6
H 2O
C 6H 6
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Empirical and Molecular Formulas Notes.notebook
January 23, 2014
But Mr. Wissmann, how do we calculate Empirical Formulas?
It's easy as 1...2...3....
(assuming you are given % composition)
Step 1: Assume you have a 100 gram sample (this makes the math easier) and convert the percent compositions to grams for each element.
Ex. 58% Oxygen in a 100 g sample is 58 g
12% Carbon in a 100 g sample is 12 g
30% Silicon in a 100 g sample is ___ g
Step 2: Convert each element from grams to moles
Step 3: Compare the amount of moles for each element and determine the mole ratio
– Divide each mole value by the smallest number of moles calculated, then round to nearest whole number (if close). Ex. 8 moles C, 12 moles H, 4 moles O
– If you cannot round to a whole number, i.e. 1.3333 or 1.5, multiply to get a whole number.
Ex. 5 moles C, 8 moles H, 2 moles N
Now let's look at each step individually! 2
Empirical and Molecular Formulas Notes.notebook
January 23, 2014
Step 1: Convert % comp to grams
38.7% Carbon
Carbon:
9.75% Hydrogen
Hydrogen:
51.55% Oxygen
Oxygen:
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Empirical and Molecular Formulas Notes.notebook
January 23, 2014
Step 2: Convert the mass of each element to moles
Carbon:
Hydrogen:
Oxygen:
Step 3: Reduce ratio of moles to nearest whole number.
Moles of Carbon:
Moles of Hydrogen:
Moles of Oxygen:
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Empirical and Molecular Formulas Notes.notebook
January 23, 2014
Another Example:
What is the empirical formula for a compound that is 12% carbon, 45% hydrogen, and 43% nitrogen? 5
Empirical and Molecular Formulas Notes.notebook
January 23, 2014
Now what about molecular formula?
• To find molecular formula, you will need the molecular mass (typically given) and the mass of the empirical formula (need to calculate)
• Determine the ratio of the molecular mass and the empirical mass
• Multiply each subscript by that ratio
Ex.
Molecular Mass=220 g, Empirical mass=110 g
P4O6
Example:
Empirical Formula: C3H4O, Molecular Mass: 168 g
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Empirical and Molecular Formulas Notes.notebook
January 23, 2014
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Empirical and Molecular Formulas Notes.notebook
January 23, 2014
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