Chemistry Reference
In-Depth Information
You may be asked to determine the molecular formula of a compound,
given the empirical formula and the molecular mass of the compound. This
type of calculation simulates one experimental procedure for determining
the molecular formula of a given compound. The key to solving this type of
problem is to find the mass of the empirical formula and divide that value
into the molecular mass, then multiply the result by each of the subscripts
in the empirical formula.
Example 1
Determine the molecular formula for a compound with an empirical
formula of CH
2
and a molecular mass of 70 u (atomic mass units).
Step 1. Use the periodic table to determine the mass of the empirical
formula.
C = 12.0 u, H
2
= 2.02 u
Mass of empirical formula = (12.0 u + 2.02 u) = 14.0 u
Step 2. Divide the molecular mass by the mass of the empirical formula.
70 u
14 u
= 5
Step 3. Multiply the result by the subscripts in the empirical formula.
5 × (CH
2
) = C
5
H
10
It is also likely that you will be asked to determine the empirical for-
mula for a compound, if you know its percentage composition by mass.
The key to solving this type of problem will be to assume that you are deal-
ing with a 100-gram sample, which will allow you to change percentages to
grams. Then divide the mass of each part by the mass values shown on the
periodic table. Finally, figure out the simple whole number ratio given by
the answers to these calculations.
Example 2
A particular compound has been experimentally determined to
contain 71.5% calcium and 28.5% oxygen by mass. Determine the
empirical formula for this compound.
Step 1. Assume that we could obtain a 100-gram sample, which allows us to
work with grams.
71.5 grams of calcium and 28.5 grams of oxygen
