1. Percent Composition and Chemical Formulas

2. Composition Composition- what a substance is made of
Ex: H2O is composed of 2 H atoms and 1 O atom

3. Law of Definite Proportions
in a compound, the elements combine in the same proportions by mass (g)
Ex: H2O H= 1 gram x 2 atoms = 2g
O= 16 grams x 1 atom = 16g
..so for every 1 mole of H2O you have 2g H & 16g O
(1 mole H2O = 18 grams total).

4. Percent Composition The relative amounts of each element in a compound
% mass of element X= grams of element X x 100
grams of compound

5. Ex: H2O H= 1 gram x 2 atoms = 2g
O= 16 grams x 1 atom = 16g
% mass H = g H x 100 = 11.1%
18 g H2O
% mass O = g O x 100 = 88.9 %
18 g H2O  

6. Example 1 Calculate the percent composition of propane (C3H8).
1. Determine the molar mass of the compound:
mass of C in 1 mol C3H8 – 3 x 12.0 g = 36.0 g
mass of H in 1 mol C3H8 – 8 x 1.0 g = g g C3H8

7. 2. Use the formula % mass = grams of element x 100
molar mass of compound
% mass C = g C x 100 = 81.1%
44.0 g C3H8
% mass H = g H x 100 = 18 %

8. Example 2
An 8.20-g piece of Mg combines completely with 5.40g of O to form a compound.
What is the compound? ________________
What is the percent composition of this compound?
Use the formula:
% mass of element X= grams of element X x 100
grams of compound
MgO

9. % mass of Mg = 8.20 g of Mg x 100 = 60.3%
13.60 g of MgO
% mass of O = 5.40 g of O x 100 = 39.7%
When you add them together, it should be 100% or very, very close to it:
60.3% % = 100%

10. Example 2
9.03g Mg combine completely with 3.48g N to form a compound.?
What is the compound? ________________
What is the percent composition of this compound?
Use the formula:
% mass of element X= grams of element X x 100
grams of compound
Mg3N2

11. Answer % mass of Mg = 9.03 g of Mg x 100 = 72.2% Mg 12.51 g of Mg3N2
% mass of N = 3.48 g of N x 100 = 27.8% N