Investigate and Identify the Law of
Conservation of Mass – I8C
Matter cannot be created or destroyed in an
ordinary chemical reaction.
To show this all chemical equations must be
balanced! When balancing an equation only
coefficients can be added or changed. Subscripts
cannot be changed!
Example:
4Fe + 3O2 → 2Fe2O3
The coefficients are in red. The chemical
formula and subscripts are in black.
To balance an equation take an “atom inventory”
on both sides of the equation:
___K + ___H2O  ___KOH + ___H2
Reactants
Products
K: 1
K: 1
H: 2
H: 3
O: 1
O: 1
Next choose coefficients to balance the equation:
_2_K + _2_H2O  _2_KOH + _1_H2
Reactants
Products
K: 1
2
K: 1
2
H: 2
4
H: 3
4
O: 1
2
O: 1
2
Practice Problems:
(All practice problems are from TEA released TAKS Tests)
1. What are the coefficients that will balance this chemical
equation?
A 2, 1, 1
B 3, 4, 2
C 2, 2, 1
D 4, 3, 2
2. The chemical equation shows CaCO3 being
heated. Which of these statements best describes
the mass of the products if 100 g of CaCO3 is
heated?
A The difference in the products’ masses is equal to
the mass of the CaCO3.
B The sum of the products’ masses is less than the
mass of the CaCO3.
C The mass of each product is equal to the mass of
the CaCO3.
D The sum of the products’ masses equals the mass
of the CaCO3.
(All practice problems are from TEA released TAKS Tests)
3. Which chemical equation supports
the law of conservation of mass?
F 2H2O(l) → H2(g) + O2(g)
G Zn(s) + HCl(aq) → ZnCl2(aq) + H2(g)
H Al4C3(s) + H2O(l) → CH4(g) + Al(OH)3(s)
J CH4(g) + 2O2(g) → CO2(g) + 2H2O(g)
(All practice problems are from TEA released TAKS Tests)
5. When the above equation is balanced, the
coefficient for magnesium chloride is —
A0
B1
C2
D4
(All practice problems are from TEA released TAKS Tests)
6. According to the law of conservation of
mass, how much zinc was present in
the zinc carbonate?
A 40 g
B 88 g
C 104 g
D 256 g
(All practice problems are from TEA released TAKS Tests)
9.
In the procedure shown above, a calcium chloride
solution is mixed with a sodium sulfate solution to
create the products shown. Which of the following is
illustrated by this activity?
(A) The law of conservation of mass
(B) The theory of thermal equilibrium
(C) The law of conservation of momentum
(D) The theory of covalent bonding
__Ca(C2H3O2)2 + __K3PO4 →
__Ca3(PO4)2 + __KC2H3O2
10. Which set of coefficients balances the equation
above?
(F) 3, 3, 1, 2
(G) 6, 1, 1, 3
(H) 3, 2, 1, 6
(J) 6, 2, 1, 6