1. The Determination of Percentage Composition
Mole Theory
The Determination of
Percentage Composition

2. Introduction
The determination of a percentage is one of the more important calculations that we do in this course.
It will appear as a calculation in its own problems.
But, it will be a component (and a useful tool) in more involved calculations in this and other units.
Bottom line….you need to be good at this one.

3. So what is a Percentage?
First, (and most importantly) a percentage is a ratio.
More specifically, it is the ratio of a part divided by a total.
Typically, that ratio will then be multiplied by 100 for reporting purposes. (since an entire system is assigned the numerical value 100 percent)

4. Mathematically, this looks like this:
Percentage =
amount of part
total of all parts x
100

5. A simple example:
Suppose that an AP Chemistry class has 16 juniors and 20 seniors. What percentage of the class is in the senior class?
Following the equation on the previous slide:
Percentage =
amount of part
total of all parts x
100
we can set up the calculation as follows:

6. Suppose that an AP Chemistry class has 16 juniors and 20 seniors
Suppose that an AP Chemistry class has 16 juniors and 20 seniors. What percentage of the class is in the senior class?
The part that we care about (in this problem) is the seniors and we are told that there are 20 of them. The total of all the parts in the system (the juniors and the seniors) is = 36.
Therefore, the calculation will set up as: 20
Percentage of seniors = x
100 36 =
55.56 %

7. However, the calculation is a bit more involved when we consider chemical compounds…
We must account for the numbers of atoms of each element in the compound – that means we need correct formulas.
We have to remember that elements have different atomic weights – we must account for this in the calculation.
Ultimately, we end up calculating a quantity called a mass percentage which accounts for the concepts described in the previous bullet points.

8. What is the mass percentage of carbon in simple sugar ?
An example:
Simple sugar has the chemical formula C6H12O6
What is the mass percentage of carbon in simple sugar ?
The chemical formula indicates that there are 6 atoms of carbon in every molecule.
The periodic chart reports the atomic mass of carbon as 12 amu.
Accordingly, we can determine the “mass” of the part (the carbon in this problem) as 6 x 12 = 72 amu of carbon.

9. What is the mass percentage of carbon in simple sugar ?
Continuing…
Simple sugar has the chemical formula C6H12O6
What is the mass percentage of carbon in simple sugar ?
Using the formula of sugar and the periodic chart, we can also calculate the total mass of the sugar molecule…
It will be (6 x 12) + (12 x 1) + (6 x 16) = 180 amu
A question for you: What do we call the calculation we just did?

10. What is the mass percentage of carbon in simple sugar ?
Finishing this up:
Simple sugar has the chemical formula C6H12O6
What is the mass percentage of carbon in simple sugar ? 72
Percentage of carbon x
100 =
180 =
40.00 % Carbon
Note that we did this calculation using amu’s as the unit of mass. We can easily “scale the calculation up” to grams if we assume I mole of the compound.