1. Law of Definite Composition and Law of Multiple Proportions

2. Joseph Proust (France 1799)
Law of Definite Proportions:
A given compound always contains elements in a certain proportion by mass. (Constant composition).

3. Law of Definite Proportions
Atoms combine in whole number ratios, so their proportion by mass will always be the same.
Example: H2O is always made up of 2 atoms of H and one atom of O.
The mass ratio of O to H in water is always 16:2 or 8:1.
Constant composition implies constant properties (i.e., water always boils at 100ºC and freezes at 0ºC)

4. Chemical compounds have the same mass ratio of elements no matter how formed
Copper Carbonate

5. Example: KCl always contains one atom of K for every one atom of Cl
In KCl, potassium and chlorine always have a mass ratio of “39.09 to 35.45” or “1.1 to 1” by mass.

6. Law of Multiple Proportions
• Compounds of differing mass ratios of the same elements are found, but they will have different properties
• Example: carbon dioxide (CO2) and carbon monoxide (CO)

7. Law of Multiple Proportions (John Dalton)
When the same two elements combine to form more than one compound:
the ratios of the mass of one element in the first compound to its mass in the second compound, (as it combines with the same mass of the other element), can always be expressed as ratios of small whole numbers

8. For example CO and CO2 Both have carbon and oxygen
Both have one carbon
The ratio of oxygen is 1:2 (CO : CO2)

9. Dalton’s Atomic Theory
• John Dalton- (1803)
Law of Multiple Proportions, Atomic Theory of Matter
Elements of Dalton’s atomic theory:
Matter is composed of atoms
Different elements have different atoms
(e.g, C has a mass=12, O has a mass=16)
Compounds form when different elements combine in fixed proportions

10. Determining the Mass of a Compound
EXAMPLE:
If C has a mass of 12 and O has a mass of 16, what is the mass of:
a) Carbon monoxide (CO)
= 28
b) Carbon dioxide (CO2)
12 + 2(16) = 44

11. Determining the Mass of a Compound
Find the mass of NaCl, NaOH, CaCO3, and H2SO4
The atomic masses for each element are found on the periodic table.
(for now use these amounts)
Na=23 Cl=35 O=16 Ca=40
C=12 H=1 and S=32

12. Example of Law of Multiple Proportions
Carbon combines with oxygen to form CO and CO2 .
Mass of Carbon(g)
Mass of Oxygen(g)
Ratio of O in CO2 to O in CO CO
12.01
16.00
CO2
32.00
2:1

13. Practice Problem 1
In the carbon compounds ethane (C2H6) and ethene (C2H4), what is the lowest whole number ratio of H atoms that react with the same number of C atoms?
Answer:
Mass of Carbon
Mass of Hydrogen
Ratio of H
C2H6
24.02 6
C2H4 4

14. Practice Problem 2
Carbon reacts with oxygen to form two compounds as shown:
Compound A: g C, g O. Compound B: g C, g O.
Find the lowest whole number ratio of O that react with an equal mass of C.

15. Solution Strategy X=.652 Y=.375
X= Y=.375
X/Y = so ratio is 1.7:1 or rounded 2:1

16. Fill in table with info Oxygen Carbon Ratio A 3.22 2.41 1.34 B 17.9
6.71
2.67
1:2

17. Practice Problem 3 Lead forms two compounds with oxygen as shown:
Compound A: 2.98 g Pb, g O.
Compound B: 9.89 g Pb, g O.
For a given mass of oxygen, what is the lowest whole number mass ratio of lead in the two compounds?

18. Fill in table Pb Oxygen ratio Compound A 2.98 0.461 Compound B 9.89
0.763
A 2.98/.461=6.46
B 9.89/.763=12.96
1:2 ratio

19. Pb
Oxygen
Ratios A
2.98
.461
6.46 B
9.89
.763
12.96
1:2 ratio
A has 1 Pb:1 O
B has 2 Pb:1 O