Chemistry XI

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
CHCl3 (b) Iron (III) sulfate Fe2 (SO4)3.
Solution
a. 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 CHCl3 = 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 (SO4)3 = 399.9 amu
The answer rounded to three significant figures is 4.00 x 102 amu.

Problems for Practice
Calculate the formula weights of the following compounds
a. NO2 b. glucose (C6H12O6) c. NaOH d. Mg(OH)2
e. methanol (CH3 OH) f. PCl3 g. K2 CO3

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