HONORS CHEMISTRY
NAME
Formula Weight / Balancing Equations Review
3-12-15
KEY
The next stop we make is to solve MASS RELATIONSHIP PROBLEMS, known in the field of chemistry as STOICHIOMETRY.
These problems will require you to apply what you know in writing formulas/naming compounds, calculating formula
weight for compound, writing and balancing equations, and some math calculations that we will learn over the next
few days.
As a REVIEW in preparation, complete the following problems ON YOUR OWN PAPER!
FORMUL WEIGHT (FW) CALCULATIONS
In formula weight, you take the NUMBER OF ATOMS of each element represented by the compound’s formula,
MULTIPLY that number by the ATOMIC MASS of the element from the periodic table, then ADD the results and ROUND
TO SIGNIFICANT FIGURES using the add/subtract rule.
The unit for FORMULA WEIGHT is g/mole.
Example: Calculate the FW of Water, H20.
2 moles H
x
1.0079 g
=
2.0158 g
1 mole O
x
15.999 g
=
15.999 g
18.0148 g/mole = 18.015 g/mole rounded to SF
Calculate the FORMULA WEIGHT of the following compounds:
1. CaSO4
1 Ca
x
40.08 g
=
40.08 g
1 S
x
32.06 g
=
32.06 g
4O
x
15.999 g
=
63.996 g
_____________
136.136 g/mole
136.14 g/mole
2. Mg3(PO4)2
3 Mg x
24.305 g
=
72.915 g
2P
x
30.974 g
=
61.948 g
8O
x
15.999 g
=
127.992 g
___________
262.855 g/mole
3. AlC2H3O2
1 Al
x
26.982 g
=
26.982 g
2C
x
12.011 g
=
24.022 g
3H
x
1.0079 g
=
2O
x
15.999 g
=
3.0237 g
31.998 g
_______________
86.0257 g/mole
86.026 g/mole
4. Na2CO3
2 Na
x
22.990 g
=
45.98 g
1 C
x
12.011 g
=
12.011 g
3 O
x
15.999 g
=
47.997 g
______________
105.988 g/mole
105.99 g/mole
BALANCING EQUATIONS
Remember that a CHEMICAL REACTION happens at the sub-microscopic level and we cannot see the actual processed
occurring. To represent a chemical reaction we write a CHEMICAL EQUATION on paper.
Reactant(s)
Product(s)
REACTANTS – one or more substances that ENTER a chemical reaction, separated by PLUS signs.
PRODUCTS – one or more substances that are PRODUCED during a chemical reaction, separated by PLUS signs.
Remember that an equation MUST BE BALANCES by placing COEFFICIENTS in front of the formulas; you CANNOT alter
subscripts.
BALANCE THESE CHEMICAL EQUATIONS
3 KOH 
1)
Na3PO4
+
2)
MgF2
3)
P4
4)
2 RbNO3
5)
2 AgNO3 + Cu 
6)
CF4
7)
2 HCN
8)
GaF3
+
+

+
K3PO4
2 LiF
2 P2O3

CuSO4
3 Cs
+
MgCO3 +
BeF2 
+
2 Br2
+

Li2CO3
+ 3 O2
3 NaOH

Be(NO3)2 + 2 RbF
Cu(NO3)2 + 2 Ag
CBr4 +

2 F2
H2SO4
3 CsF
+
+
Ga
Cu(CN)2