Percent Composition
Parts of a Whole
Percent
• Short for per centum…”by the hundred”
• Part of a whole
• If there are 10 people in a room and 4 of them
have blue eyes, what percentage of people
have blue eyes?
Composition
• Chemical formula
• Molar mass
Percent Composition
• Write the formula
• Find the molar mass
• Determine the percentage of the part in
question
Problem #1
• Determine the percent composition of water.
Problem #1—Step 1
• Write its formula
▫ H2O
Problem #1—Step 2
• Determine its molar mass
▫ H—2 x 1.01 = 2.02
▫ O—1 x 16.00 = 16.00
18.02 g/mol
Problem #1—Step 3
• Determine the portion that each element
makes up by its mass and multiply by
100…determine the percent composition
▫ H—2 x 1.01 =
2.02 x 100 = 11.2% H
18.02
▫ O—1 x 16.00 = 16.00 x 100 = 88.8% O
18.02
Problem #2
• Determine the percent composition of
aluminum carbonate.
Problem #2—Step 1
• Write its formula
▫ Al2(CO3)3
Problem #2—Step 2
• Determine its molar mass
▫ Al—2 x 26.98 = 53.96
▫ C—3 x 12.01 = 36.03
▫ O—9 x 16.00 = 144.00
233.99 g/mol
Problem #1—Step 3
• Determine the percent composition
▫ Al—2 x 26.98 = 53.96 x 100 = 23.1% Al
233.99
▫ C—3 x 12.01 = 36.03 x 100 =15.4% C
233.99
▫ O—9 x 16.00 = 144.00 x 100 =61.5% O
233.99
Problem #3
• A compound contains only copper and chloride
ions. It is found to be 47.3% copper.
Determine the formula of the compound.
Problem #3—Step 1
• Determine the possible formulas
▫ Copper can have charges of 1+ or 2+
▫ So, your choices are: CuCl and CuCl2
Problem #3—Step 2
• Determine both molar masses
▫ Cu—1 x 63.55 = 63.55
▫ Cl—1 x 35.45 = 35.45
99.00 g/mol
▫ Cu—1 x 63.55 = 63.55
▫ Cl—2 x 35.45 = 70.90
134.45 g/mol
Problem #3—Step 3
• Determine the percentage of copper in each.
▫ Cu—1 x 63.55 = 63.55 x 100 = 64.2% Cu
99.00
▫ Cu—1 x 63.55 = 63.55 x 100 = 47.3% Cu
134.45
So, the answer is CuCl2
Problem #4
• A compound is known to contain 41.7%
magnesium. Determine if the compound is
magnesium fluoride, magnesium hydroxide, or
magnesium oxide.
Problem #4—Step 1
• Determine the formulas
▫ MgF2
▫ Mg(OH)2
▫ MgO
Problem #4—Step 2
• Determine all three molar masses
▫ Mg—1 x 24.31 = 24.31
▫ F—2 x 19.00 = 38.00
62.31 g/mol
▫ Mg—1 x 24.31 = 24.31
▫ O—2 x 16.00 = 32.00
▫ H—2 x 1.01 =
2.02
58.33 g/mol
▫ Mg—1 x 24.31 = 24.31
▫ O—1 x 16.00 = 16.00
40.31 g/mol
Problem #4—Step 4
• Determine the percentage of magnesium in
each.
▫ Mg—1 x 24.31 = 24.31 x 100 =39.0% Mg
62.31
▫ Mg—1 x 24.31 = 24.31 x 100 = 41.7% Mg
58.33
▫ Mg—1 x 24.31 = 24.31 x 100 =60.3% Mg
40.31
Problem #4—Answer
• Mg(OH)2
Practice, practice, practice