The document explains the law of definite proportions, emphasizing that elements in a compound are always present in constant mass ratios. It provides examples of calculating mass percent composition for compounds, including hydrogen in water and zinc and sulfur in a given compound. Additionally, it illustrates how to determine the percent composition of elements in a chemical formula and assigns homework related to the topic.

1. PERCENT
COMPOSITION
Section 6.1

2. THE LAW OF DEFINITE
PROPORTIONS
• The elements in a chemical
compound are always present in
the SAME proportions by mass
• Ex: H2O: the proportion of
Hydrogen and Oxygen stays
constant

3. MASS PERCENT OF AN
ELEMENT
% Composition = Mass of element x 100%
Mass of compound

4. MASS PERCENT OF AN
ELEMENT
• Mass percent of hydrogen in
water:
• Mass of H2 = 2 (1.01) = 2.02 g/mol
• Mass of H2O= 18.02 g/mol (Molar
mass)
• % mass of H2 = mass of H2 100%
• mass of H2O
= 2.02 g/mol x 100%
18.02 g/mol
= 11.2 %

5. PERCENT COMPOSITION
• Tells you the mass
percent (%) of all
elements in a molecule

6. PERCENT COMPOSITION
Ex 1: A compound with a mass of
48.72 g is found to have 32.69 g
of zinc and 16.03g of sulfur. What
is the percent composition of the
compound?
Given:
Mass of compound 48.72 g
Mass of Zn = 32.69 g
Mass of S = 16.03 g

7. PERCENT COMPOSITION
• % Zn = 32.69g x 100%
48.72g
• % S = 16.03g x 100%
48.72g
= 67.10%
= 32.9%

8. PERCENT COMPOSITION
• Can also be calculated using the chemical
formula
• Ex 2: What is the % composition of each
of the elements in C9H8O?
• = 9(12.01g/mol) + 8(1.01g/mol) + 16g/mol
= 108.09 + 8.08 + 16
= 132.17g/mol
O
H
C
M 8
9

9. PERCENT COMPOSITION
• % C = 108.09 g x 100%
132.17g
= 81.78 %
• % H = 8.08g x 100%
132.17 g
= 6.11 %
• % O = 16.00g x 100%
132.17 g
= 12. 10%

10. PERCENT COMPOSITION
• Sometimes chemists use %
composition when they have
unknown compounds to
determine the mass of the
elements that make up the
compound

11. HOMEWORK
• READ Section 6.1
• Page 260: # 1-9
• Page 264: # 11-18
• Page 266: # 21-30