Percentage Composition
Today you will be completing notes on Percentage Composition.
In order to do so, you must first be able to calculate a Formula Weight.
You took notes on this last week and they are available at the counter if
you want them and turned them in.
You will also be using the problems from the end of that note sheet again
today to do the assignment; those problem are included at the end of
these notes.
Complete today’s classwork in the space provided at the end of this note
sheet and turn in your work.
Use the computer’s CALCULATOR to do the math as needed.
Percentage
Composition
An Analytical
Chemistry Technique
• I CAN calculate the PERCENTAGE
COMPOSITION of a compound from Formula
Weight data.
Percentage Composition
• An analytical chemistry technique used in
studying composition of substance is
Percentage Composition.
• In Percentage Composition, Formula Weight
(Molar Mass) data is used to determine the
overall contribution the mass of each element
makes to the mass of a compound.
Calculating Percentage Composition
• Determine the FW (MM) of the compound.
• Divide the TOTAL MASS of each element by
the FW for the compound.
• Multiply the results by 100 to convert to a
percentage.
• Add all percentages to check the answer…it
should be VERY CLOSE to 100% (may be
slightly off due to rounding).
Practice Problem
• What is the percentage composition of Magnesium
Hydroxide, Mg(OH)2 ?
• STEP 1. Calculate the FW:
•
•
•
•
•
1 mole Mg X 24.035 g
2 moles O X 15.999 g
2 moles H X 1.0079 g
You will need these TOTAL masses
for each element in the next step!
= 24.305 g/mole
= 31.998 g/mole
=
2.0158 g/mole
+ _____________
58.3188 g/mole
• (Since this isn’t the last step, don’t round to SF here)
• STEP 2. Divide EACH ELEMENT’S TOTAL MASS by the FW:
•
Mg = 24.305 g X 100 = 41.676 %
•
58.3188 g
•
•
O = 31.998 g
58.3188 g
X 100 = 54.867 %
•
•
H = 2.0158 g
58.3188 g
X 100 = 3.4565 %
• STEP 3. Add all the %s :
•
41.676 + 54.867 + 3.4565 = 99.99956 % = 100%
ASSIGNMENT
• For each of the problems that follow, use the
information in the problem to calculate the
PERCENTAGE COMPOSITION of each element!
• Complete your work in the space provided.
Use the computer’s calculator to do the math.
Problem #1
1. Aluminum Sulfate Al2(SO4)3
2 moles Al
3 moles S
12 moles O
X
X
X
26.982 g
32.06 g
15.999 g
=
=
=
53.964 g/mole
96.18 g/mole
191.988 g/mole
+ ________________
345.132 g/mole
345.13 g/mole
Problem #2
2. Glucose, C6H12O6
6 moles C X
12 moles H X
6 moles O X
12.011 g/mole =
1.0079 g/mole =
15.999 g/mole =
72.066 g/mole
12.0984 g/mole
95.994 g/mole
+ _______________
180.1584 g/mole
180.158 g/mole