1. Introduction to laws in chemistry
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Dr Seemal Jelani Chem-100

2. Lavoisier: The Law of Conservation of Mass
INTRODUCTION
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Dr Seemal Jelani Chem-100

3. What Do You See? Look closely! What is its purpose?
When the pans are both at the same height, what does that mean if you have masses on both pans?
Keep this in mind as we continue
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Dr Seemal Jelani Chem-100

4. Law of conservation of mass
LCM can neither be created nor destroyed
Matter is converted from one kind to another during a chemical reaction
Atoms are neither created nor destroyed during chemical change
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Dr Seemal Jelani Chem-100

5. Chemical Reactions involve a rearrangement of atoms into different combinations
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Dr Seemal Jelani Chem-100

6. Lavoisier: The Law of Conservation of Mass
Early 1700’s Lavoisier: Law of Conservation of Mass
During a chemical change, matter is neither created nor destroyed.
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Dr Seemal Jelani Chem-100

7. LAW OF CONSERVATION OF MASS
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In every chemical operation an equal quantity of matter exists before and after the operation
That is, the amount of matter before a reaction must equal the amount of matter after a reaction
No matter is lost.
The total mass of reactants = total mass of products
2/23/2019
Dr Seemal Jelani Chem-100

8. Conclusion
Matter is neither created nor destroyed during a chemical reactions but changes from one form to another
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Dr Seemal Jelani Chem-100

9. LAW OF CONSERVATION OF MASS
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When g of magnesium was heated in air, g of magnesium oxide (MgO) was produced.
a) what is the mass of oxygen needed to produce g MgO?
2/23/2019
Dr Seemal Jelani Chem-100

10. Total mass reactants = total mass products
Using the LCM:
Total mass reactants = total mass products
mass of Mg + mass O = mass of MgO
g Mg + mass O = g MgO
mass O = g = g O
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Dr Seemal Jelani Chem-100

11. Proust: The Law of Definite Proportions
1799, Proust: Law of Definite Proportions
A compound always contains the same elements in certain definite proportions.
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Dr Seemal Jelani Chem-100

12. Water
Chemical analysis revealed that as long as it is pure i.e. its composition is always one mole of oxygen to two moles of hydrogen
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Dr Seemal Jelani Chem-100

13. LAW OF DEFINITE PROPORTIONS
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In a compound, the ratios by mass of the elements in that compound are fixed independent of the origins or preparation of that compound
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Dr Seemal Jelani Chem-100

14. CaCO3
NaOH
H2SO4
KMnO4
NH3
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Dr Seemal Jelani Chem-100

15. A compound is unique because of the specific arrangement and weights of the elements which make up that compound
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Dr Seemal Jelani Chem-100

16. That is, elements combine in whole numbers
Also it is not possible to have a compound with portion an atom.
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Dr Seemal Jelani Chem-100

17. 2/23/2019
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Dr Seemal Jelani Chem-100

18. LAW OF DEFINITE PROPORTIONS
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Elements combine in specific ratios to form compounds
Use the Generic equation for percent:
% = ( portion / total ) 100
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Dr Seemal Jelani Chem-100

19. What is the experimental percent of oxygen in CO2 if 42
What is the experimental percent of oxygen in CO2 if 42.0 g of carbon reacted completely with g of oxygen?
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Dr Seemal Jelani Chem-100

20. % O = (mass of O / mass of CO2) 100
% O= [112.0 g O / (42.0 g g) CO2] 100
= 72.7% O
2/23/2019
Dr Seemal Jelani Chem-100

21. % Al = (Atomic mass of Al / Formula mass of Al2O3) 100
2 What is the theoretical percent of aluminum in aluminum oxide?
% Al = (Atomic mass of Al / Formula mass of Al2O3) 100
% Al = (54 amu / 102 amu) 100 = 52.9%
3. What is the percent composition of sodium chloride?
% Na = 39.3% % Cl = 60.7%
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Dr Seemal Jelani Chem-100

22. Conclusion
All pure samples of a particular compound contain similar elements combined in the same proportion by mass
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Dr Seemal Jelani Chem-100

23. LAW OF MULTIPLE PROPORTIONS
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LAW OF MULTIPLE PROPORTIONS
When two elements form a series of compounds, the masses of the one element that combine with a fixed mass of the other element stand to one another in the ratio of small integers.
Iron oxide exists in different ratios with different properties
FeO and Fe2O3
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Dr Seemal Jelani Chem-100

24. If two elements A and B, combine to form more than one chemical compound, then the various masses of one element, A which combine separately with a fixed mass of the other element, B are in a simple multiple ratio
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Dr Seemal Jelani Chem-100

25. Examples
Copper and oxygen combine to give black copper(II) oxide, CuO and red copper(I) oxide Cu2O
Iron and oxygen combine to give brown iron(II), Fe2O3 and black iron(I) oxide, FeO
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Dr Seemal Jelani Chem-100

26. Law of conservation of mass & Law of definite proportions
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When g of magnesium was heated in air, g of magnesium oxide (MgO) was produced.
b) what is the percent of Mg in MgO?
% Mg = (mass Mg / Mass MgO) 100
= (0.0976g / g) 100
= 60.3 %
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Dr Seemal Jelani Chem-100

27. Using only LDP, what mass of oxygen was needed to combine with the magnesium?
% O = 100% MgO % Mg = 39.7% O
% O = (mass O / mass MgO) 100
39.7 % = (mass O / g) 100
mass O = ( g) = g O
Same as using the LCM!!
2/23/2019
Dr Seemal Jelani Chem-100