1. Our problem for the day…
What is the percent composition of Calcium Chloride??

2. But first…what exactly is percent composition?
In chemistry class we define it as “the percentage by mass of each element in a compound.”
The mass percentage of an element in a compound is the same regardless of the sample’s size

3. Percent Composition Formula
Mass of element in compound
% element in compound
X 100 =
Mass of Compound

4. Class Example:
Calculate the percent composition by mass of each element in calcium chloride: CaCl2

5. Your first step is to find the Atomic masses of the elements in the compound…look at the periodic table

6. Your Second Step is to determine the total masses of each element and of the compound.
To make things easier, we are going to round the atomic masses to the nearest tenth of a gram. So Calcium’s atomic mass is 40.1grams and Chlorine’s atomic mass is 35.5 grams!
Now we add the Calcium and Chlorine together BUT because we have two chlorines we have to multiply Chlorine’s atomic mass by 2!
Ca (40.1 grams) + Cl2 (71.0 grams) = grams for the mass of CaCl2

7. Last step: This is the big one!
Mass of calcium (Ca)
% calcium in compound
X 100 =
Mass of calcium chloride (CaCl2)
40.1 grams
36.1 % calcium in compound
X 100 =
111.1 grams

8. Can you calculate the chlorine on your own??
Mass of chlorine (Cl)
% chlorine in compound
X 100 =
Mass of calcium chloride (CaCl2)
71.1 grams
63.9 % calcium in compound
X 100 =
111.1 grams