Empirical and
Molecular Formula
Miss Knick
HAHS
Empirical Formula
O Is the simplest whole number ratio of the
atoms in a compound
Practice: Empirical or Not…is the question.
CH4
P2O5
C 2H 6
P4H10
Calculating the empirical
formula
1. Convert the mass of each element to moles
2. Divide each number of moles by the smallest
number of moles of all the answers to #1
3. If the answers to #2 are whole numbers, these are
the subscripts in the empirical formula
If any of the answers are not whole numbers,
covert all answers to a common fraction.
Practice (skip 8 lines between
each)
O Ex #1: The compound that gives vinegar its sour
taste is acetic acid, which contains the elements
carbon, hydrogen, and oxygen. When 5.00grams
of the compound is analyzed, it is found to
contain 2 g of carbon, 0.336 g of hydrogen, and
2.66 g of oxygen. What is the empirical formula?
O Ex. #2: A compound contains 36.5% sodium,
25.4% sulfur, and 38.1% oxygen. What is the
empirical formula?
Molecular Formula
O The true ratio of the atoms in a compound
as it exists naturally; may be the same as
the empirical formula; will relate to the
empirical formula in whole number ratios
Calculating Molecular
Formula
O 1. Find the molar mass of the empirical
formula.
O 2. Divide the molecular formula mass by the
empirical formula mass.
O 3. Multiple each subscript in the empirical
formula by the answer to #2, these are the
subscripts for the molecular formula
Mass Spectrometer
7
Tro: Chemistry: A Molecular Approach, 2/e
Practice
O Ex #3: The molar mass of acetic acid has
been determined to be 60 g/mol. Using that
information with the simplest formula
calculated in Ex #1, determine the
molecular formula of acetic acid.