Chemical
Calculations : Formula Weight (FW) or Formula Mass
The formula weight of a substance is the
sum of the atomic weights of all atoms in a formula unit of the compound,
whether molecular or not.
Sodium chloride, NaCl, has a formula
weight of 58.44 amu (22.99 amu from Na plus 35.45 amu from Cl). NaCl is ionic,
so strictly speaking the expression "molecular weight of NaCl" has no
meaning. On the other hand, the molecular weight and the formula weight
calculated from the molecular formula of a substance are identical.
Solved
Problem
Calculate the formula weight of each of
the following to three significant figures, using a table of atomic weight
(AW):
(a) chloroform CHCl(3)
(b) Iron (III) sulfate Fe2 (SO(4))3.
Solution
(a)
chloroform CHCl(3)
1
x AW of C = 12.0 amu
1 x AW of H = 1.0 amu
3 x AW of Cl = 3 x 35.45 = 106.4
amu
Formula weight of CHCl(3) = 119.4
amu
The answer rounded to three significant
figures is 119 amu.
(b)
Iron (III) sulfate
2 x Atomic weight of Fe = 2 x 55.8 =
111.6 amu
3 x Atomic weight of S = 3 x 32.1 =
96.3 amu
3 x 4 Atomic weight of O =12x16 = 192.0 amu
Formula weight of Fe2 (SO(4))3 = 399.9 amu
The answer rounded to three significant
figures is 4.00 x 102 amu.