Percent Composition (Section 11.4)

1
Percent Composition(Section 11.4)

• Helps determine identity of unknown compound
• Think CSIthey use a mass spectrometer
• Percent by mass of each element in a compound

2
To calculate percent composition

• H2O
• Use molar masses to calculate mass of compound
• 2 x 1.0 1 x 16.0 18.0 g/mol
• Use molar masses to calculate mass of each
  individual element
• H 2 x 1.0 2.0 g/mol
• O 1 x 16 16.0 g/mol
• Calculate using the formula
• H (2 / 18) x 100 11.1 Hydrogen
• O (16/18) x 100 88.9 Oxygen
• (notice to check your work 11.1 88.9 100)

3
Percent Composition of Water

• H (2 / 18) x 100 11.1
• O (16/18) x 100 88.9
• Even though there are 2 hydrogen atoms in water,
  most of the mass of water comes from the 1 oxygen
  atom

4
Percent Composition Practice

• KNO2 Percent composition of N
• Molar mass of N 1 x 14 14 g/mol
• Molar mass of compound 1 x 39.1 1 x 14 2 x
  16 85.1 g/mol
• by mass of N (14 / 85.1) x 100 16.45

5

• Calculate the percent composition
• Phosphorus in Al2(PO4)3
• P 3 x 31.0 93 g/mol
• Al2(PO4)3 2 x 27 3 x 31 12 x 16 339 g/mol
• P (93 / 339) x 100 27.43
• Chlorine in LiCl
• Cl 1 x 35.5 35.5 g/mol
• LiCl 1 x 6.9 1 x 35.5 42.4 g/mol
• Cl (35.5 / 42.4) x 100 83.73
• Calcium in CaSO4
• Ca 1 x 40.1 40.1 g/mol
• CaSO4 1 x 40.1 1 x 32.1 4 x 16 136.2
  g/mol
• Ca (40.1 / 136.2) x 100 29.44

6

• Calculate the percent composition
• Sodium in NaCl
• Na 1 x 23 g/mol 23 g/mol
• NaCl 1 x 23 1 x 35.5 58.5 g/mol
• Na (23 / 58.5) x 100 39.32
• Sulfur in H2SO4
• S 1 x 32.1 32.1 g/mol
• H2SO4 2 x 1 1 x 32.1 4 x 16 98.1 g/mol
• S (32.1 / 98.1) x 100 32.72
• Potassium in KNO3
• K 1 x 39.1 39.1 g/mol
• KNO3 1 x 39.1 1 x 14 3 x 16 101.1 g/mol
• K (39.1 / 101.1) x 100 38.67

7
Empirical vs Molecular Formulas

• Empirical formula formula with smallest whole
  number ratio of elements
• Molecular formula formula with actual number of
  atoms of each element
• Empirical Molecular
• C4H9 C8H18
• H2O H6O3
• We use the percent composition to calculate the
  empirical and molecular formulas.

8
How to calculate empirical formula

• Use mass of elements to calculate moles
• If given percentages, assume sample is 100 grams.
  Therefore, percent by mass will equal the actual
  mass of element.
• Pick which element has the smallest number of
  moles
• Divide each element by the smallest number of
  moles to get ratio (these numbers become
  subscripts)
• Write empirical formula

9

• A blue solid is found to contain 36.43 nitrogen
  and 63.16 oxygen. What is the empirical
  formula?
• If we assume sample is 100 grams, we have 36.43
  g N and 63.16 g O.
• Calculate moles of each element
• N 36.43 / 14 2.60 mol
• O 63.16 / 16 3.95 mol
• Smallest number of moles nitrogen with 2.60 mol
• Divide by smallest number of moles
• N 2.60 / 2.60 1 x 2 2
• O 3.95 / 2.60 1.5 x 2 3
• we must get whole numbers as our answerthats
  why we multiplied by two
• Empirical formula N2O3

10

• Determine the empirical formula for a compound
  that contains 35.98 aluminum and 64.02 sulfur
• If we assume sample is 100 grams, we have 35.98
  g Al and 64.02 g S.
• Calculate moles of each element
• Al 35.98 / 27 1.33 mol
• S 64.02 / 32.1 2.00 mol
• Smallest number of moles aluminum with 1.33 mol
• Divide by smallest number of moles
• Al 1.33 / 1.33 1 x 2 2
• S 2.00 / 1.33 1.5 x 2 3
• we must get whole numbers as our answerthats
  why we multiplied by two
• Empirical formula Al2S3

11

• Find the empirical formula for a substance that
  contains 48.64 carbon, 8.16 hydrogen, and
  43.20 oxygen
• If we assume sample is 100 grams, we have 48.64
  g C, 8.16 g H, and 43.20 g O.
• Calculate moles of each element
• C 48.64 / 12 4.05 mol
• H 8.16 / 1 8.16 mol
• O 43.20 / 16 2.70 mol
• Smallest number of moles oxygen with 2.70 moles
• Divide by smallest number of moles
• C 4.05 / 2.70 1.5 x 2 3
• H 8.16 / 2.70 3.0 x 2 6
• O 2.70 / 2.70 1.0 x 2 2
• we must get whole numbers as our answerthats
  why we multiplied by two
• Empirical formula C3H6O2