Transcript Law of Conservation of Mass

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During this class, you will use the
Law of Conservation of Mass to
identify patterns in chemical
equations.
•
Think about the following question:
– If you burned a log in the fireplace,
would the mass of the products
(smoke, ashes, etc.) be the same as
the reactants (the wood)?
– “What does it mean to conserve to
you?”
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Yes!
In a chemical reaction, the total mass of
the reactants is always equal to the total
mass of the products
Matter is conserved  type of atoms
does not change
– Nothing is created or destroyed
• Example:
Methane
Burning of Methane
+ Oxygen → Water +
Carbon dioxide

CH4 + O2 → H2O + CO2
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CH4 + O2 → H2O + CO2
4 Hydrogen atoms on the left hand side
(reactants); only 2 on the right side
(products)
Where did they go?
Atoms cannot be created or destroyed, just
rearranged in a chemical reaction
NEED TO BALANCE THE EQUATION!

• The
CH4 + 2 O2 → 2 H2O + CO2
2 means that there are 2 oxygen
and water molecules
• Now there are equal numbers of atoms
on both sides of the reaction
1.If
50 grams of sodium reacts with
chlorine to form 126 grams of salt. How
many grams of chlorine reacted?
 50g
Sodium + ____Chlorine 126 g
Salt
Answer: 126g-50g= 76g of Chlorine
If 20 grams of aluminum reacts with 200
grams of bromide to form aluminum
bromide, and no aluminum is left after
the reaction, but 23 grams of bromine
remained unreacted. How any grams of
aluminum bromide were formed?
• 20g Aluminum + 177g Bromide ____g
Aluminum Bromide
• Answer: 177g+20g= 197 g of aluminum
bromide
•

Determine if the equations are balanced or
unbalanced. Justify.

AgNO3 + H2S
 Ag2S + 2HNO3

•
If 178.8 g of water is separated into
hydrogen and oxygen gas, and the
hydrogen gas has a mass of 20.0 g.
What is the mass of the oxygen gas
produced?
•
From a laboratory process, a student
collects 28.0 g of hydrogen and 224.0 g
of oxygen. How much water was
originally involved in the process?
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A student carefully placed 23.0 g of
sodium in a reactor with an excess
quantity of chlorine gas. When the
reaction is complete, the student
obtained 58 grams of salt. How many
grams of chlorine gas reacted?
•
A 10 gram sample of iron reacts with
oxygen to form 18.2 grams of ferric
oxide. How many grams of oxygen
reacted?


Determine if the equations are balanced or
unbalanced. Write Yes or No. give a short
justification for each answer.
3Zn(OH)2
Mg(ClO3)2
+

2 H3PO4  Zn3(PO4)2 + 3 H2O
MgCl2
+
O2
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Why is it important for a chemist to
understand the law of conservation of
mass?
“If you burned a log in the fireplace, would
the mass of the products (smoke, ashes,
etc.) be the same as the reactants (the
wood)?”
•
During this class period, you used the law of conservation of
mass to identify patterns in chemical equations. (DOK 1, 2a)
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What is the Law of Conservation of Mass?
Explain the difference between a balanced and
unbalanced equation.
Tomorrow, you will (identify the properties of elements and
describe the structure of the periodic table) identify
unbalanced chemical equations and describe and apply steps
to balance them. (DOK 1, 2a)