Molecular Formulas
Section 11.4
Caffeine
Caffeine is known medically as trimethylxanthine, and the
chemical formula is C8H10N4O2
It’s important
caffeine is an
Among its many
operates using
mechanisms that
cocaine, and
to know that
addictive drug.
actions, it
the same
amphetamines,
heroine use to
stimulate the brain. Relatively speaking, caffeine's effects are
milder than amphetamines, cocaine and heroin, but it is
manipulating the same channels in the brain, and that is one of
the things that gives caffeine its addictive qualities.
Molecular Formulas
The final step in identifying an unknown compound is determining the
molecular formula, which shows the actual number of atoms in each
molecule of the substance. To determine the molecular formula, the molar
mass of the compound must be determined through experimentation.
For example, the molar mass of acetylene (C2H2) is 26 g/mol
The empirical formula for acetylene is CH and the molar mass of the
empirical formula is 13 g/mol
Therefore, if the molar mass of the molecular formula for acetylene is divided
by the molar mass of the empirical formula for acetylene
26 g/mol ÷ 13 g/mol = 2
The resulting value of “2” shows that the molecular mass must contain twice
as the number of carbon atoms and twice the number of hydrogen atoms as
the empirical formula.
C2H2
How to Find Molecular Formulas
1. calculate the empirical formula
2. use the equation : (empirical formula mass)x = molar mass
3. find value for x:
x = molar mass/empirical formula mass
4. multiply each subscript in empirical formula by value for x
How to Find Molecular Formulas
Example:
Given that the empirical formula of a compound is CH and the
molar mass is 104 g/mol, calculate the molecular formula.
mass of C = 12.0 g/mol
mass of H = 1.01 g/mol
empirical formula mass = 13.0 g/mol
to find the number of CH units in the compound:
# CH units = (104 g/mol)/(1 mol/13.0 g) = 8.00
Molecular formula = 8(CH) or C8H8
Steps to Determine Molecular
Formulas
1.Express percent by mass in grams.
2.Determine the number of moles of each element.
3.Write the empirical formula. Be sure to examine the mole ratio
for whole numbers.
4.Determine the integer that relates the empirical and molecular
formulas by dividing the experimental molar mass by the mass of
the empirical formula.
5.Multiply the subscripts by the answer to step # 4.
6.Write the molecular formula.
Molecular Formula Practice I
1. Analysis of a chemical used in photographic developing fluid
indicates a chemical composition of 65.45% C, 5.45% H, and
29.09% O. The molar mass is found to be 110 g/mol. Determine
the molecular formula.
2. A compound was found to contain 49.98 g carbon and 10.47 g
hydrogen. The molar mass of the compound is 58.12 g/mol.
Determine the molecular mass of the formula.
3. A colorless liquid composed of 46.68% nitrogen and 53.32%
oxygen has a molar mass of 60.01 g/mol. What is the molecular
formula?
Hydrates
A hydrate is a compound that has a specific number of water
molecules bound to its atoms. To analyze hydrates, the water of
hydration must be driven off by heating the compound. When
the water of hydration is driven off, an anhydrous solid is
produced. The anhydrous form of a hydrate (dessicator) is used
to absorb water. Anhydrous compounds can be packaged with
materials that need to be kept moisture free.
Example: Hydrated Silica
Hydrated silica is the abrasive used in gel toothpastes, or in the clear portion of striped
toothpastes. It is often used in opaque toothpastes as well, as a secondary abrasive.
When dried in an oven, hydrated silica looses the water, and becomes a dessicant, a
substance that attracts water from the air. You find little packets of "silica gel" crystals
in containers whose contents will be damaged by condensing moisture, such as
vitamin bottles, consumer electronics, pepperoni, or leather products.
Steps to Determining Anhydrous
Formulas
1. Determine the number of moles of the
compound.
2. Determine the number of moles of water.
3. Divide by the smallest number of moles.
4. Determine the ratio.
Examples of Hydrate Problems
1. What is the mole ratio of the hydrate
CuSO4 ·____ H2O, if the anhydrous mass
of the CuSO4 is 25.42 g and the mass of
the water is 14.3 g?
Examples of Hydrate Problems
Continued
2. What is the mole ratio of the hydrate
Na2CO3 · ____ H2O, if the original mass of
the hydrated Na2CO3 was 43.69 g and the
mass of the anhydrous Na2CO3 is 16.19
g?
Naming Hydrates
Prefix
Molecules
H 20
Formula
Name
Mono-
1
(NH4)2C2O4 · H20
Ammonium oxalate
monohydrate
Di-
2
CaCl2 · 2H20
Calcium chloride dihydrate
Tri-
3
NaC2H3O2 · 3H20
Sodium acetate trihydrate
Tetra-
4
FePO4 · 4H20
Iron (III) phosphate
tetrahydrate
Penta-
5
CuSO4 · 5H20
Copper (II) sulfate
pentahydrate
Hexa-
6
CoCl2 · 6H20
Cobalt (II) chloride
hexahydrate
Hepta-
7
MgSO4 · 7H20
Magnesium sulfate
heptahydrate
Octa-
8
Ba(OH)2 · 8H20
Barium hydroxide
octahydrate
Deca-
10
Na2CO3 · 10H20
Sodium carbonate
decahydrate