Download Chemical Formula Analysis

Chemical Formula Analysis
Types of Chemical Formulas
Empirical formula – the simplest whole number
ratio of elements in a compound
Ex) C2H6, empirical formula is __________________
Molecular – indicates the actual number of
atoms of each element in one molecule of a
compound
Ex) C6H12O6
Types of Chemical Formulas
(continued)
Structural – represents the arrangement of
atoms in a molecule
% Composition of elements in a
compound
Def: the percent by mass of each element that
makes up a compound
Procedure:
1. Calculate the molar mass of compound
2. find the % by mass, by dividing the total
mass of the element by the molar mass
of the compound
% Composition of elements in a
compound
% mass = molar mass of element x # of atoms presentx100
molar mass of cmpd
Ex) Determine the percent composition of H2O.
Hydrates
Definition: compounds that contain water
molecules trapped in their crystal lattice
structure
Ex) MgSO4  7 H2O (Epsom salts)
- dot represents the physical separation of
water molecules from the ionic compound in
which they are trapped
Hydrates (continued)
• We can calculate the % by mass of water in a
hydrate by:
1. Calculating the total formula mass of
compound.
2. divide the mass of the water in the hydrate
by the mass of the compound
Determining Empirical Formula from
Percent Composition
• Problem: Determine the Empirical formula for
a compound that has is 70.9% potassium and
29.1% sulfur by mass.
Procedure:
Step 1: Assume the percent to be the # of
grams per 100 g of the compound.
Therefore, we assume 70.9 g K and 29.1 g S
Determining the Empirical formula
(cont’d)
Step 2: Find the # of moles of each element.
Divide by molar mass.
K = (70.9 g)
(39.1 g/mol)
S = (29.1g)
(32.0 g/mol)
K = 1.82 mol
S = 0.91 mol
Still going!
Step 3: Divide the moles of each element by the
smallest of the values
K = 1.82 moles
0.91
S = 0.91 moles
0.91
K=2
S=1
The end is near!
You’ve found the simplest whole # ratio of the
elements that make up the empirical formula,
now use them as subscripts!
K2S
YaY!!!!
Determining Molecular formula from
Percent composition
Do the same procedure for finding the empirical
formula, there is just one more step.
Problem: A certain compound was found to
consist of 80.0% carbon and 20.0% hydrogen,
with a molecular formula mass of 30.0 g.
Determine the molecular formula for the
compound.
How do we attack this thing?
Step 1: Determine the empirical formula for the
compound (like we just did, refer to your
notes)
CH 3
Here’s the twist!
Step 2: Determine the empirical formula mass
mass = 15 g
Almost there!
Step 3: Divide the molecular formula mass
(from question) by the empirical formula
mass, to find the multiple of the empirical
formula
30.0 g = 2
15.0 g
Finish it up!
Step 4: multiply the subscripts of the empirical
formula by the result of your last step
(CH3)2
C2H6