Molecular Formulas - Brookwood High School

Molecular Formulas
Section 11.4
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
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.
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
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 C H
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