3­ Empirical Formula Notes.notebook
Notes on Empirical Formulas
Empirical Formulas
simplest (whole number) mole ratio
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can be found or determined by experimental data
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How to find empirical formulas from mass data
Example Problem:
A compound consisting of Pt and Cl is 26.7% by mass Cl.
Write the empirical formula.
Info
Mass Ratio
Cl
Pt
26.7%
73.3%
(CHANGE % TO GRAMS - since 100% is accounted for)
Mole Ratio
Simplify
by smallest mole #
26.7gCl x I mole Cl
35.5gCl
73.3gPt x 1 mole Pt
195.1gPt
=0.376 moles Pt
=0.752 moles Cl
0.376 = 1
0.376
0.752 = 2
0.376
empirical formula =PtCl2
LAST STEP
1. If very close to whole # then round (1.9 = 2
2. If not close to then do a 2nd simplifying step
1 to 1.5
x2
x2
2 : 3
1 to 1.33
x3
x3
3 : 4
4.1 = 4)
1 to 1.25
x4
x4
4 : 5
Apr 15­1:07 PM
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3­ Empirical Formula Notes.notebook
Notes on finding Molecular Formulas
To find the molecular formula:
You need the following:
Empirical Formula (simplest mole ratio)
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Molar Mass
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Example:
CH2 is empirical formula
Molar mass is 42 grams/mole
A.
TRIAL AND ERROR
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Ø
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B.
If CH2 is the real formula - then molar mass = 14 g/mol
If C2H4 is the real formula - then molar mass = 28 g/mol
If C3H6 is the real formula - then molar mass = 42 g/mol
(empirical formula mass) x = (molar mass)
14x = 42
x=3
multiply x by all subscripts in the empirical formula
emp. formula ratio = 1:2 C=3 then H=6
Molecular formula = C3H6
Apr 16­8:41 AM
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