Conserving Mass in a Chemical Reaction and Balancing
In a chemical equation, the number of each atom on the reactants side has to equal the number of that
same atom on the products because a chemical reaction must obey the Law of Conservation of Mass.
This law states: Matter________________________________________________________. Or more
simply, the mass of substances produced (products) is always equal to the mass of the reacting
substances (reactants).
“What goes in must come out", although it may be in a different form (state, compound) the amount of
mass of each atom is the same. *(The number of each elements on the reactants side has to equal the
same number on the products side)*
Ex.
C(s) + O2(g) -----> CO2(g)
C:
O:
Is this equation balanced from Reactants to Products side?
C:
O:


In order to balance a chemical equation, we must place ____________ (big numbers out in front
of chemical formulas) . You cannot change subscripts or add numbers anywhere in a formula
except for the front. DO NOT CHANGE formulas!
Coefficients apply to ______________ in a chemical formula.
Rules for Balancing Chemical Equations
1. Write out the skeleton equation (unbalanced) and make sure that you have written the formula
for each molecule correctly.
2. Balance atoms that occur in the largest number first. Leave oxygen and hydrogen until the end.
3. Balance polyatomic ions next (only if they are the same on both sides) ex: SO42- must be equal
on both sides.
4. Balance hydrogens and oxygens last.
5. Write the state of each molecule or element.
6. Check your answer! All coefficients should be in lowest terms.
ex.
H2
ex.
Mg
+
+
O2 ---->
O2 ---->
H2O
MgO
ex.
CH4 +
ex.
O2
Zn +
-----> H2O
AgNO3 ---->
Zn(NO3)2 +
Ag
More Balancing
KI +
Pb(NO3)2 ---->
CaSO4
HCl
+
AlBr3
+
C3H8 +
Ba3(PO4)2
Ca(OH)2
O2 ----->
+
KNO3
------>
------>
CO2
KBr --->
+
+
PbI2
Al2(SO4)3
H2O
+
+
CaBr2
CaCl2
H2O
K3PO4)
+
BaBr2