Chemical Calculations : Formula Weight (FW) or Formula Mass

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.