Standard H.C.3: The student will demonstrate an understanding of the structures and classification of chemical compounds.
H.C.3A.7 Analyze and interpret data to determine the empirical formula of a compound and the percent composition of a compound.
Our Target:
Appetizer:
You will be able to define and find the molecular and empirical formulas given the masses of elements that make up a compound.
Compare the two molecular compounds below:
What are the ratios of atoms for each molecule?
What would the molecular formulas be?
Acetylene
What is the same between the two and what is different?
Benzene
KEY TERMS
Empirical Formula - Smallest whole number ratio of atoms in a compound
Molecular Formula - “True Formula” - the actual number of atoms in a compound
Example:
ethane
Propene
Molecular
formula =
C2H4
C3H6
To find the empirical formula, simply reduce the molecular formula to
the lowest whole number ratio.
C2H4
C3H6
Reduce
Lewis structure =
Empirical
formula =
CH3
CH3
CH3
CH3
*KEY POINT: Compound that have different molecular formulas may
have the same empirical formulas.
General guidelines for determining Empirical Formula
1. Determine # of moles of each element in the formula.
- if given mass of element, simple divide by molar mass of element to get # of moles.
2. Divide moles by the smallest # to find subscripts.
- if given % of each element, assume you are dealing with a 100g sample.
3. When necessary, multiply subscripts by 2, 3, or 4 to get whole #’s.
Ex1 - Find the empirical formula for a sample that contains
297.2 g of Fe and 127.8 g of O.
- convert to moles
- divide moles by the smallest # to find subscripts.
Ex2 - Find the empirical formula for a sample of 25.9% N and
74.1% O.
- convert the % into masses
- convert to moles
- divide moles by the smallest # to find subscripts.
P1 – Find the empirical formula for a sample that contains 2.67 g of
carbon and 7.11 g of oxygen.
P2 – Determine the Empirical formula for the 400 gram sample
containing 31.5% cobalt, 34.28% sulfur, and 34.22% oxygen.
Determining Molecular Formula from the empirical formula
1. Find the empirical formula.
3. Divide the molecular mass by the empirical mass.
2. Find the empirical formula mass. 4. Multiply each subscript by the answer from step 3.
Ex3 - The empirical formula for ethylene is CH2. Find the
molecular formula if the molecular mass is 28.1 g/mol?
P3 – The empirical formula for a common drying agent is P2O5. The
molecule has a molar mass of 283.88 g/mol. Find the molecular
formula of the compound.