Percent Composition

Percent Composition refers to the percentage by mass. does not tell us the relative rate by which the atoms combine, or what portion of the volume is made up by an individual element.

Calculating Percentage Composition Look at two examples of word problems. Example 1 Jack has a dozen of doughnuts. 3 of these doughnuts are chocolate. What percentage of doughnuts are chocolate?

How would you figure this out?

% of doughnuts that are chocolate = Number of chocolate doughnuts x 100 Total number of doughnuts Answer: 3 x 100= 25% chocolate doughnuts 12

Example 2 Jim goes to a school that has 258 total students.  If 94 of the students are girls, what percentage of the student body is made up of girls?

% of the student body that is made up by girls = Number of girls x 100 Total number of students                                                                           

% of the student body that is made up by girls = Answer % of the student body that is made up by girls = 94 x 100= 36.4% girls 258

    In both cases, you found the percent composition by dividing one part by the total and multiplying by 100.

Calculating Percent Composition percent composition = part x 100 whole

Finding the percentage composition by mass works the same way Finding the percentage composition by mass works the same way.  You divide the mass that one element contributes to the compound by the mass of the entire compound. 

Stoichiometry is the study of the quantitative relationships that can be derived from chemical formulas and from chemical equations.

Formula Mass Is the sum of the atomic masses of all the atoms represented by the formula of the substance Molecular Mass- another name for the formula mass of a molecular substance.

The formulas of all chemical compounds tell… The elements present in a compound The relative number of atoms of each element in a compound 3. The number of atoms of each element present in 1 molecule of the substance.

Water Molecule Percentage composition of water shows what percentage of the mass of a water molecule is made up of hydrogen and what percentage of the mass is made up of oxygen. 

Finding Formula Mass of H2O The subscripts show that for every 2 Hydrogen atoms, there is 1 Oxygen atom. What is the formula mass of H2O? Formula mass= sum of atomic masses of 3 atoms.

Answer H= 1.0u O=16u Answer: 1.0x2= 2.0 +16.0= 18.0u

What percentage of the mass of carbon dioxide (CO2) is made up by the carbon? first find the mass of the total compound. C = 12.0 u x 1 atom  = 12.0 u O = 16.0 u x 2 atoms = 32.0 u                                    --------                                      44.0 u next use the formula: Partial mass from carbon x 100  total mass of the CO2                                                                                          12.0 u x 100 = 27.3% carbon 44.0 u                                                                                   

What is the percentage composition of glucose (C6H12O6) ? find the mass of the entire molecule 2. Use the formula to calculate the percent composition of each element

Formula Mass C = 12.0 u x 6 atoms   = 72.0 u H = 1.01 u x 12 atoms = 12.1 u O = 16.0 u x 6 atoms   = 96.0 u                                    ----------                                       180 u

Answer 72.0 u % for Carbon = ---------------- x 100 = 40.0% 180 u                                12.1 u % for Hydrogen = ------------ x 100 = 6.7 %                                180 u                               96.0 u % for Oxygen = ----------------  x 100  = 53.3%                               180 u