Section 6.3
Formulas of Compounds
Objectives
1. Recognize and explain the differences between
empirical and molecular formulas
2. Calculate the empirical formula of a compound from
experimental data or percentage composition data.
Section 6.3
Formulas of Compounds
Empirical Formula: Examples
Compound
Hydrogen peroxide
Water
Ethane
Tetrachloroethylene
Molecular Formula
Empirical Formula
Section 6.3
Formulas of Compounds
A. Empirical Formulas
• The empirical formula of a compound is the simplest
whole number ratio of the atoms present in the
compound.
• It may or may not be the exact molecular formula of a
compound
• The empirical formula can be found from the percent
composition of the compound.
Section 6.3
Formulas of Compounds
B. Calculation of Empirical Formulas
Section 6.3
Formulas of Compounds
B. Calculation of Empirical Formulas
Finding the empirical formula from experimental data
Question:
Determine the empirical formula of a compound
containing 5.75 g Na, 3.5 g, N, and 12.0 g O.
1. Convert masses of each element into moles.
Section 6.3
Formulas of Compounds
B. Calculation of Empirical Formulas
2. Find mole ratios by dividing the # moles of each
element by the smallest # of moles.
3. Write the empirical formula.
Section 6.3
Formulas of Compounds
B. Calculation of Empirical Formulas
Finding the empirical formula from percent composition
Question:
Find the empirical formula for a compound that has the
following mass percentages: C – 23.54 %, H –
1.98%, and F-74.48 %.
1. Convert percentages to maass (assume 100g sample)
Section 6.3
Formulas of Compounds
B. Calculation of Empirical Formulas
2. Follow the same procedure from the previous
problem.
Section 6.3
Formulas of Compounds
C. Calculation of Molecular Formulas
Objective:
Determine the molecular formula for an unknown
compound from its empirical formula and molar mass.
Remember:
An empirical formula is the simplest whole number
ratio of atoms of each element that make up a
compound.
Section 6.3
Formulas of Compounds
C. Calculation of Molecular Formulas
A molecular formula gives the actual number of atoms
of each element in a compound.
• A molecular formula is always a whole number
multiple of an empirical formula
For example:
Ethane (C2H6) is 2 X its empirical formula (CH3)
• Therefore, the molar mass of a compound is a whole
number multiple of the molar mass of the empirical
formula
Section 6.3
Formulas of Compounds
Sample Problem:
The empirical formula of a compound is C3H4NO3. The
compound has a molar mass of 408 g/mol. What is the
molecular formula of the compound?
1. Find the molar mass of the compound.
2. Divide the molar mass of the compound by the molar
mass of the empirical formula.
3. Multiply the empirical formula by the result from #2
and write molecular formula.
Section 6.3
Formulas of Compounds
Formula Summary
For the sugar glucose