1. Percent Composition

2. Percent Composition
The gram formula mass of a compound represents the TOTAL MASS of the compound
What percent of the total mass is each element in the compound?
Remember: percent means “out of 100”

3. For example…. This cup contains a total mass of 100 g of liquid.
75 g are “coffee” and 25 g are
“cream”.
What is the percent composition of the liquid in the cup?
75g coffee x = 75% coffee
100g total
25g cream x = 25% cream
100g total

4. What is the percent composition of potassium chlorate?
Formula = KClO3
Gram Formula Mass : K = 1 x = 39
Cl = 1 x = 35.5
O = 16 x = 48
+______
122.5 g
What percent of the total mass (122.5 g) is each of the
elements (K, Cl, and O) in the compound?

5. K = 39 g
39 g x = %
122.5 g
35.5 g x = %
122.5 g
Cl = 35.5 g
O3 = 48 g
48 g x 100 = %
122.5 g
+ ________
100 %

6. Percent composition formula
is in Table T (back page)
mass of part x 100
mass of whole

7. You try it! What is the percent composition of MgBr2?
Mg = 24 g x 1 = 24 g
Br = 80 g x 2 = 160g
+_____
184 g
Mass of whole
% Mg g (mass of part) x = %
184 g
% Br g x = %
184 g

8. % Composition and Hydrates
Same as other compounds, except treat the “water” as a part:
What is the % composition of water in the hydrated compound Na2CO3 • 10 H2O

9. Na2CO3 • 10 H2O Total mass (GFM) = 286 g
Mass of water (the “part”) = 10 x 18g = 180 g
% composition of water = g x = % water
286 g

10. You try it!
What is the percent composition of water and calcium sulfate in the hydrated compound
CaSO4 • 2H2O
21 % water
79 % calcium sulfate