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Law of
Conservation of
Mass
and
Balancing Chemical
Equations
Let’s Review Counting Atoms
Copy the following chemical formulas in your
notes and count the TOTAL number of atoms.
1.
2.
3.
4.
5.
4 K2SO4
(NH4)PO4
Ba(OH) 2
2 KNO4
2 Ca3(PO4)
2
________
________
________
________
________
Father of Modern
Chemistry
1743 - 1794
First Described the “Law of Conservation of Mass”
Conservation of Mass
Antoine Lavoisier found that the mass of the
reactants and the products are equal, even
when the states of matter change.
HgO
He started with:
10g of Mercury
Oxide (HgO)
Hg + O2
He ended up with:
...But what
happened to
the O2?
and 9.3g Mercury…
10 g. = 0.7 + 9.3 g
This is the Law of the
Conservation of Mass
It states that: “Matter is
neither created or destroyed,
it only changes form.”
• These ‘changes’ include phase changes
solid
liquid
gas
• The amount that you started with will be equal
to the amount you end with
MASS REACTANTS = MASS PRODUCTS
Lavoisier’s Chemical Equation:
2HgO

2 Hg
+ O2
Hg (mercury) can exist by itself...but, oxygen will need to bond
with another oxygen to make O2
To balance the atoms we need to:
Put the coefficient of 2 in front of reactant HgO.
Put the coefficient of 2 in front the product Hg.
This balances the equation!
Easy, Right?
Not that easy?
Let’s try...
BALANCING CHEMICAL
EQUATIONS
Four Steps to Balance
Equations:
1. Set up your equation.
Write the metals, nonmetals, oxygen, and
hydrogen.
2. Count and then write the number of atoms
you have on both sides.
3. Balance by changing the coefficients and
recounting.
4. Start the process again if it still does not
balance.
Before we begin balancing,
we will look at a couple of rules.
1.You can only change the
coefficients, and NOT the
subscripts.
2.The coefficients must be
whole numbers.
(Ex. – 1, 2, 3, 4…)
CLASS PRACTICE
Copy these in your notes.
____
H2 +
____
____
Mg +
____
Na +
O2 
____
____
____
HCl 
HCl 
H2O
____
____
H2+
____
MgCl2
NaCl +
____
H2
Let’s kick it Up a Notch!!
Copy these in your notes
__ KClO3  __KCl + __O2
__ Na + __ H20  __ NaOH + __ H2
__ S8 + __ O2  __SO3
Independent Practice
1. __Fe + __ O2  __Fe2 + __O3
2. __Cl2 + __Li  __LiCl
3. __ ZnS + __ O2  __ZnO + __SO2
4. __ Cl2 + __ NaI  __ NaCl + __ I2