1. The Law of Conservation of Mass
Lesson 1 1

2. Understanding Chemical Equations
What is a substance?
A substance is an element or a compound
The substances 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

3. Counting Elements and Atoms
Glucose
Elements
A chemical that cannot be broken down into another substance
Each unique capital letter represents a different element.
How many elements are in glucose?
C6H12O6
3 Elements
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4. Counting Elements and Atoms
Adding the Subscripts
How can you tell there is only one atom of oxygen in H2O?
Subscripts
The small numbers after the element’s name
No subscript means there is only one atom present. (The one is not needed.)
Molecule of an element (same kind of atoms) H2
Molecule of an compound (different kind of atoms)
H2O
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5. Subscripts
C12H22O11
There are 12 atoms of Carbon
There are 22 atoms of Hydrogen
There are 11 atoms of Oxygen
If there is not a subscript listed, it is understood to be 1.
Example: NaCl
There is one atom of Sodium
There is one atom of Chlorine

6. Determine how many of each element are in the problems on your notes.6

7. NaHCO3 HCl Group Practice! Hydrogen – 1 Chlorine - 1 Sodium – 1
Carbon – 1
Oxygen -3
HCl
Hydrogen – 1
Chlorine - 1

8. CuSO4 C2H4O2 NH3 CaO4 C4H10 MgBr2 Let’s practice counting.
Count the number of atoms in each formula using the subscript.
Check your answer with a partner.
CuSO4 C2H4O2 NH3 CaO4 C4H10 MgBr2
© KeslerScience.com

9. Counting Elements and Atoms
Subscripts + Coefficients
Big number at the beginning of this formula is a coefficient
Involved in counting atoms
Represents the number of molecules for that compound
Important when balancing equations
2H2SO4
Sulfuric Acid
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10. Counting Elements and Atoms
Calculating the number of atoms including the subscripts and coefficients. This formula means there are 2 H2SO4 molecules. We know in each molecule there are: 2 hydrogens 1 sulfur 4 oxygens
2H2SO4
Sulfuric Acid
To find the total number of atoms, multiply each of these atoms by 2.
Total atoms for this formula = 14
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11. 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
Understanding this simple concept is critical to your ability to balance equations properly.

12. 3H3PO4 2H2O Group Practice!! Hydrogen – 9 Hydrogen – 4 Phosphorus – 3
Oxygen - 12
2H2O
Hydrogen – 4
Oxygen - 2

13. Determine how many of each element are in the problems on your notes.13

14. 6CuSO4 2C2H4O2 3NH3 5CaO4 4C4H10 4MgBr2 Let’s practice counting.
Count the total number of atoms for each element in the compounds below.
Check your answer with a partner.
6CuSO4 2C2H4O2 3NH3 5CaO4 4C4H10 4MgBr2
© KeslerScience.com

15. There are times you will see a compound with parenthesis.
Pb(NO3)2
The 2 after the parenthesis indicates there are two sets of the parenthesis.
Pb(NO3) (NO3)
So, in counting the atoms, you would have the following:
Lead – 1 Oxygen - 6
Nitrogen -2

16. Counting Elements and Atoms
Calculating the number of atoms including the parenthesis. This formula means aluminum is bonded to a molecule of sulfur and oxygen. We know in this molecule there are: 2 aluminums 3 sulfurs 12 oxygens
Al2(SO4)3
Aluminum sulfate
To find the total number of atoms, multiply 3 by each element in the parenthesis.
Total atoms for this formula = 17
© KeslerScience.com

17. Determine how many of each element are in the problems on your notes.17

18. Ba(NO3)3 Co(NO3)2 Fe(OH)2 (NH4)2SO4 Al2(SO4)3 Al2(CO3)3
Let’s practice counting.
Count the total number of atoms in each compound using
subscripts, coefficients, and parentheses.
Check your answer with a partner.
Ba(NO3)3 Co(NO3)2 Fe(OH)2 (NH4)2SO4 Al2(SO4)3 Al2(CO3)3
© KeslerScience.com

19. Ba(NO3)3 Co(NO3)2 Fe(OH)2 (NH4)2SO4 Al2(SO4)3 Al2(CO3)3
Let’s practice counting.
Count the total number of atoms in each compound using
subscripts, coefficients, and parentheses.
Check your answer with a partner.
Ba(NO3)3 Co(NO3)2 Fe(OH)2 (NH4)2SO4 Al2(SO4)3 Al2(CO3)3
© KeslerScience.com

20. Counting Elements and Atoms
Calculating the number of atoms including subscripts, coefficients, and parenthesis. This formula means there are 2 molecules of Al2(SO4)3. We know in this molecule there are: 4 aluminums 6 sulfurs 24 oxygens
2Al2(SO4)3
Aluminum sulfate
To find the total number of atoms, multiply the 3 by each element in the parenthesis. Then multiply each element by 2.
Total atoms for this formula = 34
© KeslerScience.com

21. Determine how many of each element are in the problems on your notes.21

22. 3(NH4)3PO4 6Mg(OH)2 Group Practice!! Nitrogen – 9 Magnesium – 6
Hydrogen – 36
Phosphorus – 3
Oxygen – 12
6Mg(OH)2
Magnesium – 6
Oxygen – 12
Hydrogen – 12

23. The Law of Conservation of Mass
Lesson 2 23

24. Chemical Equations
Chemical equations express what is happening in a chemical reaction using symbols.
2K + Cl KCl
Reactant Product
yields

25. Law of Conservation of Mass
* In a chemical reaction, matter cannot be created or destroyed. It can only be changed.
* Therefore, the reactants must equal the products for the equation to be balanced.

26. What does that mean??
The reactants MUST contain the same elements and the same number of each element as the product. BUT they CAN be in different compounds.

27. Understanding Coefficients
Equations without coefficients are known as skeleton equations
Na Cl2  NaCl (unbalanced)
reactants
product
Since mass is directly related to the number of particles, if mass does not change, then the number of particles does not change.
The Law of Conservation of Mass

28. 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.

29. 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.

30. 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.

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

32. 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.

33. 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.

34. Do I Need to Know How to Balance?
NOT YET. You need to be able to recognize if the chemical reaction follows the law of conservation of mass.
Na + O Na2O
Mg + 2HCl MgCl2 + H2
NOPE
Yes!!

35. You Practice!!! HgO Hg + O2 Hg = Hg = O = O = N2 + 3H2 2NH3
Zn + 2HCl ZnCl2 + H2
NOPE
Yes
Yes

36. More Practice K + Br2 2KBr 2Fe + O2 6Fe2O3 2Na + 2H2O 2NaOH + H2 NOPE
Yes

37. N2 + O2  N2O5 ___ N ___ 2 2 ___ O ___ 2 5
Balancing from Scratch
Now that we fully understand what it means to be balanced, we will learn how to balance an equation from the beginning. Do NOT panic. You will see that while it can take some time, the steps are not hard.
Balance the following equation.
N O2  N2O5
Reactants side Products side
___ N ___ 2 2
___ O ___ 2 5

38. Reactants side Products side
Balance the equation
Al + O2  Al2O3
Reactants side Products side
___ Al ___
___ O ___

39. Reactants Products side
Balance the equation
Na + O2 → Na2O
Reactants Products side
___ Na ___
___ O ___

40. Reactants side Products side
Balance the equation
HgO → Hg + O2
Reactants side Products side
___ Hg ___
___ O ___

41. Reactants side Products side
Balance the equation
N O2  N2O5
Reactants side Products side
___ N ___
___ O ___

42. Reactants side Products side
Balance the equation
K + Br2 → KBr
Reactants side Products side
___ K ___
___ Br ___

43. Reactants side Products side
Balance the equation
Fe + O → Fe2O3
Reactants side Products side
___ Fe ___
___ O ___

44. Pb(NO3)2 + NaI  NaNO3 + PbI2 ___ Pb ___ 1 1 ___ I ___ 1 2 ___ Na ___
Balancing from Scratch
Now lets go through another one together.
Balance the following equation.
Pb(NO3) NaI  NaNO PbI2
Reactants side Products side
___ Pb ___ 1 1
___ I ___ 1 2
___ Na ___ 1 1
___ NO3 ___ 2 1

45. Balancing from Scratch
When you are asked this on a multiple choice test, you will most likely be asked in the following way:
Al HCl  AlCl H2
When the above equation is balanced, the coefficient for aluminum chloride is –
A 1
B 2
C 3
D 4
It is important that you understand that you can never know just one coefficient. You must make sure that the entire equation is balanced before you can know the answer.

46. Balancing from Scratch
When you are asked this on a multiple choice test, you will most likely be asked in the following way:
Al HCl  AlCl H2
When the above equation is balanced, the coefficient for aluminum chloride is –
A 1
B 2
C 3
D 4
___ Al ___ 1 1
___ H ___ 1 2
___ Cl ___ 1 3
Step 1: Determine how many of each element are present. (Remember that it is often easier to think of polyatomic ions as 1 substance.)

47. Balancing from Scratch Hydrogen and chlorine are unbalanced!
When you are asked this on a multiple choice test, you will most likely be asked in the following way:
Al HCl  AlCl H2
When the above equation is balanced, the coefficient for aluminum chloride is –
A 1
B 2
C 3
D 4
___ Al ___ 1 1
Hydrogen and chlorine are unbalanced!
___ H ___ 1 2
___ Cl ___ 1 3
Step 2: Identify an element that is keeping the equation from being balanced. Just do one at a time.

48. 2 ___ Al ___ 1 1 1 1 ___ H ___ 2 1 2 2 ___ Cl ___ 2 1 3 3
Balancing from Scratch
When you are asked this on a multiple choice test, you will most likely be asked in the following way:
Al HCl  AlCl H2 2
When the above equation is balanced, the coefficient for aluminum chloride is –
A 1
B 2
C 3
D 4
___ Al ___ 1 1 1 1
___ H ___ 2 1 2 2
___ Cl ___ 2 1 3 3
Step 3: Add coefficient(s) to make this element balanced.

49. Balancing from Scratch
When you are asked this on a multiple choice test, you will most likely be asked in the following way:
Al HCl  AlCl H2 2
When the above equation is balanced, the coefficient for aluminum chloride is –
A 1
B 2
C 3
D 4
Notice that we have a problem. There is not a coefficient that will multiply times 3 to equal 2. Chlorine cannot be balanced this way which means the first coefficient we added was incorrect!
___ Al ___ 1 1
___ H ___ 2 2
___ Cl ___ 2 3

50. Balancing from Scratch
When you are asked this on a multiple choice test, you will most likely be asked in the following way:
Al HCl  AlCl H2 2
When the above equation is balanced, the coefficient for aluminum chloride is –
A 1
B 2
C 3
D 4
Balancing Tip: When you are experiencing troubles like this, it often makes things easier if you place a 2 in front of the compound containing the odd number of atoms.
___ Al ___ 1 1 2 1
___ H ___ 1 1 2 2
___ Cl ___ 1 1 3 6
Repeat Step 3: It is okay to make a mistake and start over. This will happen many times as you first begin balancing.

51. Balancing from Scratch
When you are asked this on a multiple choice test, you will most likely be asked in the following way:
Al HCl  AlCl H2 6 2
When the above equation is balanced, the coefficient for aluminum chloride is –
A 1
B 2
C 3
D 4
___ Al ___ 1 1 2 2
___ H ___ 6 1 2 2
___ Cl ___ 6 1 6 6
Steps 4 & 5: Recalculate how many of EVERY element you now have in the equation and continue to add coefficients until it is balanced.

52. Balancing from Scratch
When you are asked this on a multiple choice test, you will most likely be asked in the following way:
Al HCl  AlCl H2 6 2 3
When the above equation is balanced, the coefficient for aluminum chloride is –
A 1
B 2
C 3
D 4
___ Al ___ 1 1 2 2
___ H ___ 6 6 6 2
___ Cl ___ 6 6 6 6
Steps 4 & 5: Recalculate how many of EVERY element you now have in the equation and continue to add coefficients until it is balanced.

53. Balancing from Scratch
When you are asked this on a multiple choice test, you will most likely be asked in the following way: 2
Al HCl  AlCl H2 6 2 3
When the above equation is balanced, the coefficient for aluminum chloride is –
A 1
B 2
C 3
D 4
___ Al ___ 1 2 2 2
___ H ___ 6 6 6 6
___ Cl ___ 6 6 6 6
Steps 4 & 5: Recalculate how many of EVERY element you now have in the equation and continue to add coefficients until it is balanced.

54. Balancing from Scratch Don’t forget to answer the question!
When you are asked this on a multiple choice test, you will most likely be asked in the following way: 2
Al HCl  AlCl H2 6 2 3
When the above equation is balanced, the coefficient for aluminum chloride is –
A 1
B 2
C 3
D 4
___ Al ___ 2 2
Don’t forget to answer the question!
___ H ___ 6 6
___ Cl ___ 6 6
Step 6: STOP ADDING AND CHANGING COEFFICENTS once it is balanced, and check your answers when you are done!

55. Balancing from Scratch
Sometimes you might get coefficients that balance, but are not the correct set. 4 2
LiI Pb(NO3)2  LiNO PbI2 2 4 2 2
___ Pb ___ 2 1 2 1
These coefficients are NOT correct because even though they are balanced, they are not the smallest numbers that can be used. Notice that they can all be reduced by a factor of 2.
___ I ___ 4 2 4 2
___ Li ___ 4 2 2 4
___ NO3 ___ 4 2 4 2
Now the chemical equation is balanced with the correct set of coefficients. Always make sure that your coefficients are in their most reduced form.

56. The Law of Conservation of Mass
Lesson 3 56

57. 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

58. Antoine-Laurent de Lavoisier
The Law of Conservation of Mass states that the sum of the reactant’s masses is equal to the sum of the products’ masses
This is how we can solve for the mass of an unknown and why we balance chemical equations.
Antoine-Laurent de Lavoisier
The Father of Modern Chemistry

59. Reactants → Product H2 + O2 H2O The Law of Conservation of Mass.
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.
In an equation, the reactants will equal the products
Reactants → Product
H2 + O H2O

60. Very Important Information:
In the lab, mass is measured in
______ or __________.
grams
kilograms
Mass is measured using either a __________ or a _____________.
Electronic balance
Triple beam balance

61. 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.
2H O H2 O
If 4.0 grams of hydrogen are mixed with oxygen gas to make 36.0 grams of water, how much oxygen gas was used.
4.0 g
? g
36.0 g
Make sure you place the numbers with the appropriate substance.
36.0 g – 4.0 g = 32.0 grams

62. Solve for the mass of the substances on your notes.
64.0 g
? g
88.0 g
14.0 g
6.0 g
? g 62

63. 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 support 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

64. 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

65. 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.
Na Cl NaCl
If 11.5 grams of sodium are mixed with chlorine gas to make 29.3 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

66. Solve for the unknown masses in your notes.66

67. 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

68. 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.