Tuesday, April 30th: “A” Day
Wednesday, May 1st: “B” Day
Agenda
Homework Questions/Collect
Continue Sec. 7.3: “Formulas & Percentage
Composition”
In-Class Assignment: Practice pg. 245: 1-3
Homework:
Pg. 61 worksheet: 1 a-e
Concept Review
Must Show Work!
Homework Questions/Problems
Practice pg. 243: #1-4
Worksheet pg. 56: 1a-d
Molecular Formulas are Multiples of
Empirical Formulas
Molecular formula: a chemical formula that
shows the number and kinds of atoms in a
molecule, but not the arrangement of the
atoms.
A molecular formula is a whole-number
multiple of the empirical formula.
The molar mass of any compound is equal to
the molar mass of the empirical formula times a
whole number, n.
n (empirical formula) = molecular formula
Comparing Empirical and Molecular
Formulas
Compound
Formaldehyde
Acetic Acid
Empirical
Formula
CH2O
CH2O
Glucose
CH2O
Molecular Formula
CH2O
C2H4O2
2X the empirical
formula
n = 2
C6H12O6
6X the empirical
formula
n = 6
Rules for Determining a Molecular
Formula from an Empirical Formula
1. Use the periodic table to find the molar mass of
the empirical formula.
2. Divide the experimental molar mass by the
empirical molar mass. (Hint: the bigger # goes
on TOP) This number is the multiplier, n.
3. Finally, multiply the empirical formula by the n
value to get the molecular formula.
Determining a Molecular Formula from an Empirical
Formula
(Sample Problem H, pg. 245)
The empirical formula for a compound is P2O5. Its
experimental molar mass is 284 g/mol. Determine
the molecular formula of the compound.
1. Use the periodic table to find the molar mass of
the empirical formula:
P: 2 (30.97) = 61.94 g/mol
O: 5 (16.00) = 80.00 g/mol
Molar mass of P2O5 = 141.94 g/mol
Sample Problem H, continued…
2. Find the multiplier, n:
n = experimental molar mass of compound
molar mass of empirical formula
 n = 284 g/mol = 2
141.94 g/mol
Hint: the bigger #
always goes on top!
3. To find the molecular formula, simply multiply
the empirical formula by 2:
2 (P2O5) =
P4O10
Example #1
Determine the molecular formula for the
following:
Molar mass: 232.41 g/mol
Empirical formula: OCNCl
1. Find molar mass of empirical formula:
O = 16.00 g/mol
C = 12.01 g/mol
N = 14.01 g/mol
Cl = 35.5 g/mol
=
77.52 g/mol
Additional Practice
2. Find the multiplier, n:
n = experimental molar mass of compound
molar mass of empirical formula
n = 232.41 g/mol = 3
77.52 g/mol
3. To find the molecular formula, simply
multiply the empirical formula by 3:
3 (OCNCl) =
O3C3N3Cl3
Example #2
Determine the molecular formula from the
following information:
Experimental Molar Mass: 120.12 g/mol
Empirical Formula: CH2O
1. Find molar mass of empirical formula:
C:
12.01 g/mol
H: 2 (1.01 g/mol) = 2.02 g/mol
O:
16.00 g/mol
Molar Mass of empirical formula: 30.03 g/mol
Example #2 cont…
2. Find the multiplier, n:
n = experimental molar mass of compound
molar mass of empirical formula
n = 120.12 g/mol = 4
30.03 g/mol
3. To find the molecular formula, simply
multiply the empirical formula by 4:
4 (CH2O) =
C4H8O4
In-Class Assignment/Homework
You Must SHOW WORK!
In-Class Assignment:
Practice pg. 245: #1-3
Homework:
Pg. 61 Worksheet: 1 a-e
We will finish this section next time…