1. The Law of Conservation of Mass1

2. Antoine-Laurent de Lavoisier
Antoine-Laurant de Lavoisier was an 18th century French scientist and nobleman who is considered the father of modern chemistry. He recognized and named oxygen and hydrogen, was one of the scientists who developed the metric system and through his experiments confirmed that although matter may change its form, the total mass remains constant. Unfortunately, because he was a nobleman he was beheaded during the French Revolution.
Antoine-Laurent de Lavoisier
The Father of Modern Chemistry

3. Very Important Information:
In the lab, mass is measured in
______ or __________.
grams
kilograms
Mass is measured using either a __________ or a __________.
scale
balance

4. So how did Lavoisier prove this?
The idea that the total mass of matter remains constant is know as
The Law of Conservation of Mass.
It can also be stated as:
Mass is never created or destroyed.
So how did Lavoisier prove this?

5. Lavoisier’s Instruments By using very precise instruments!

6. Reactant 1 + Reactant 2 Product
According to the Law of Conservation of Mass, the mass of the reactants should equal the mass of the products.
Reactant 1 + Reactant Product
Which scientist’s results below best supports the
Law of Conservation of Mass?
Scientist
Mass of Reactant 1
Mass of Reactant 2
Mass of Product
Scientist A
19 g
22 g
42 g
Scientist B
19.4 g
22.4 g
41.7 g
Scientist C
19.36 g
22.37 g
41.74 g
Scientist D g g g

7. The Trouble With Gases
Many chemical reactions give off a gas. Because of this, the mass of the remaining solid and liquid products is less than the original mass of the reactants. Before Lavoisier, many scientists thought this was proof that mass was destroyed.

8. The Trouble With Gases
Lavoisier understood what was happening, and correctly hypothesized that the difference in mass was equal to the mass of the gas given.
So how did he prove it?
Lavoisier developed a device to trap the gases that were given off, then he performed many experiments where he showed that the mass of products, including the gas, was equal to the original mass of the reactants.

9. Understanding Chemical Equations
What is a substance?
A substance is an element or a compound
The substance that originally react, that is to say that they gain, lose or share electrons, are known as the reactants.
The new substances that are produced are known as the products.
Reactants
Products

10. Solving for Mass
When all but one of the masses are known, solving for the mass of the unknown takes the most elementary of algebra.
3.4g + ?g = 4.2g 2.1g + 8.5g = ?g g + ?g = 12.0g
? = _____ ? = _____ ? = _____
?g = 4.2g – 3.4
?g = 12.0g – 3.4g
0.8 g
10.6 g
8.6 g
2.2g + ?g = 5.6g + 2.7g g + 6.7g = ?g + 3.3g
? = _____ ? = _____
?g = 5.6g + 2.7g – 2.2g
?g = 1.5g + 6.7g – 3.3g
6.1 g
4.9 g

11. Hydrogen + Oxygen Water
Solving for Mass
When this concept is shown with a chemical equation, nothing changes about how it should be solved.
Hydrogen Oxygen Water
16.0 g g ? g
16.0 grams of hydrogen reacts with 128 grams of oxygen. Assuming all of the reactants are used up, how much water will be produced?
16.0 g g = 144 g

12. Make sure you place the numbers with the appropriate substance.
Solving for Mass
When you are solving for masses, the most important thing to pay a attention to is the placement of the given masses. Sloppiness and laziness will make an easy question turn into a wrong answer.
2Na Cl NaCl
If 11.5 grams of sodium are mixed with chlorine gas to make 29.9 grams of sodium chloride, how much chlorine gas was used?
11.5 g
? g
29.9 g
Make sure you place the numbers with the appropriate substance.
29.9 g – 11.5 g = 18.4 grams

13. Understanding Coefficients
Coefficients multiply through the entire formula of a compound.
2CO2 =
2C and 4 O
3H2O =
6 H and 3 O
4CaBr2 =
4Ca and 8 Br
5Na2O =
10 Na and 5 O

14. Understanding Coefficients
Equations without coefficients are known as skeleton equations
These equations may or may not represent the correct ratio of moles that interact during the reaction shown.
Since mass is directly related to the number of particles, if mass does not change, then the number of particles does not change.

15. Understanding Coefficients
To illustrate the Law of Conservation of Mass, we place coefficients in equations to show that mass is not created or destroyed.
These equations are called “balanced equations” because they show the same number of elements on each side.

16. Yes, this equation is balanced.
Understanding Coefficients
You can determine whether or not equations are balanced using a simple accounting method and the skill you just learned.
Is the following equation balanced?
4 Fe + 3 O2  2 Fe2O3
Reactants side Products side
___ Fe ___ 4 4
___ O ___ 6 6
Yes, this equation is balanced.

17. No, this equation is not balanced.
Understanding Coefficients
Is the following equation balanced?
Al + O2  Al2O3
Reactants side Products side
___ Al ___ 1 2
___ O ___ 2 3
No, this equation is not balanced.

18. Yes, this equation is balanced.
Understanding Coefficients
You can determine whether or not equations are balanced using a simple accounting method and the skill you just learned.
Is the following equation balanced?
2 K + 2 H2O  2 KOH + H2
Reactants side Products side
___ K___ 2 2
___ H ___ 4
2+2 4
___ O ___ 2 2
Yes, this equation is balanced.

19. Al4C3 (s) + H2O (l) → CH4 (g) + Al(OH)3 (s)
Understanding Coefficients
Is the following equation balanced?
Al4C3 (s) + H2O (l) → CH4 (g) + Al(OH)3 (s)
Reactants side Products side
___ Al ___ 4 1
___ C ___ 3 1
___ H ___ 2 7
___ O ___ 1 3
No, this equation is not balanced.

20. No, this equation is not balanced.
Understanding Coefficients and Balancing
Determining if an equation is balanced or not is a very simple task, but once again carelessness will cause an incorrect answer. Do NOT pretend to understand better than you really do. Make a table and get it RIGHT!
Is the following equation balanced?
N O2  N2O5
Reactants side Products side
___ N ___ 2 2
___ O ___ 6 5
No, this equation is not balanced.

21. Yes, this equation is balanced.
Understanding Coefficients
Is the following equation balanced?
For some of you, it will be easier to just ignore the polyatomic ions and focus on the other elements.
2LiI Pb(NO3)2  2LiNO PbI2
Reactants side Products side
___ Li ___ 2 2
___ I ___ 2 2
___ Pb ___ 1 1
Yes, this equation is balanced.

22. _____ Li _____ _____ N _____ 6 6 3 3 6 6 6 3 2 4 2 4 2 2 2 1
When you are given sets of coefficients, all you need to do is plug them in to see if they are balanced or not. Once again, this is a test of your patience more than anything else.
___Li ___N2 ___Li3N
A 6, 2, 2
B 3, 1, 2
C 3, 2, 1
D 6, 1, 2
_____ Li _____
_____ N _____ 6 6 3 3 6 6 6 3 2 4 2 4 2 2 2 1

23. Always check every answer choice!
When you are given a molecule to place into the equation, the same principle applies.
C6H12O6
H2O2
2H2O
2CO
CH O CO ____
_____ C _____
_____ H _____
_____ O _____
A 2CO
B H2O2
C 2H2O
D C6H12O6 1 7 1 1 1 1 3 1 4 12 4 4 4 4 2 4 8 4 4 4 4 4 4
Always check every answer choice!

24. _____ C _____ _____ H _____ _____ O _____ 1 1 4 4 2
When you are given a molecule to place into the equation, the same principle applies.
However, in this case there is actually a short cut!
CH O CO ____
_____ C _____
_____ H _____
_____ O _____
A 2CO
B H2O2
C 2H2O
D C6H12O6
2C + 2O 1 1
2H + 2O 4
4 H’s
4H + 2O 4 2
2 O’s
6C + 12H + 6O
Third, determine which answer meets this need.
Second, figure out what more you need.
First, add up everything that is already there.