1. Ch. 7-3 Percent Composition

2. POINT > Define percent composition POINT > Calculate percent composition POINT > Use molar mass as a conversion unit (review)

3. POINT > Define percent composition
Percent composition is the percentage, by mass, of each element in a compound
The Law of Definite Proportions tells us that the percent composition of a chemical compound does not vary from sample to sample

4. POINT > Calculate percent composition
Percent composition can be calculated from the chemical formula of a compound using molar masses:
mass of element
in 1 mol of compound
% comp
of element =
100% x
molar mass of compound

5. POINT > Calculate percent composition
Ex. Calculate the percent composition of water
Molar mass of H2O = 18.0 g/mol
Molar mass of O in 1 mol H2O = 16.0g/mol
Molar mass of H in 1 mol H2O = 2.0g/mol
% mass
of O
16.0g/mol = x
100%
= 88.9% O
18.0g/mol
11.1% H

6. WB CHECK:
Calculate the percent composition of methane:
Molar mass of CH4 = 16.0 g/mol
Molar mass of C in 1 mol CH4 = 12.0g/mol
Molar mass of H in 1 mol CH4 = 4.0g/mol
25.0% H % C

7. POINT > Calculate percent composition
If the formula is unknown, percent composition can be calculated from experimental data:
% comp of
element
mass of element (g) =
100% x
mass of compound (g)

8. POINT > Calculate percent composition
Ex. An unknown compound contains only sodium and chlorine. Analysis shows that a 25.0g sample has g Cl. What is the percent composition of the compound?
15.2g Cl
25.0 g compound
39.2% Na
x 100% = 60.8% Cl

9. WB CHECK:
A compound is formed when 1.90g of hydrogen combine with 23.1g of carbon and 27.0g of nitrogen. What is the percent composition of this compound?
3.65% H % C % N

10. POINT > Calculate percent composition
If you know the percent composition, you can use it to determine the mass of an element in a compound
percent comp of element Z
mass of
element Z
mass of
compound = x

11. POINT > Calculate percent composition
Ex. Calculate the mass of oxygen in 45.0g of water. Water is 88.9% O 45.0g H2O x .889 O = 40.0g Oxygen

12. WB CHECK:
Calculate the mass of carbon in 87.3g of methane.
65.5g Carbon

13. POINT > Use molar mass as a conversion unit (review)
How many molecules in 77.5g of methanol (CH3OH) 77.5g x = 2.42 mol CH3OH 2.42 mol CH3OH x = 1.46 x 1024 molecules CH3OH
1 mol
32.0g
6.02 x 1023 molecules
1 mol

14. Homework
Read 7.3 pages Practice #1-2 page 227 Practice #1-3 page 230 Practice #1-3 page 232 F.A. #1-6 page 232 % Comp W.S.