The Foundations of Chemistry

FORMULA WEIGHTS, MOLECULAR WEIGHTS,

AND MOLES

The formula weight (FW)of a substance is the sum of the atomic weights (AW)

of the elements in the formula, each taken the number of times the element occurs.

Hence a formula weight gives the mass of one formula unit in atomic mass units.

Formula weights, like the atomic weights on which they are based, are relative masses.
The formula weight for sodium hydroxide, NaOH, (rounded off to the nearest 0.1 amu)
is found as follows.

Number of Atoms of Stated Kind Mass of One Atom Mass Due to Element

1
Na 1
23.0 amu 23.0 amu of Na
1
H  1
0 1.0 amu  0 1.0 amu of H

1
O  1
16.0 amu 16.0 amu of O

Formula weight of NaOH40.0 amu

The term “formula weight” is correctly used for either ionic or molecular substances.
When we refer specifically to molecular (nonionic) substances, that is, substances that ex-
ist as discrete molecules, we often substitute the term molecular weight (MW).

EXAMPLE 2-7 Formula Weights

Calculate the formula weight (molecular weight) of acetic acid (vinegar), CH 3 COOH, using
rounded values for atomic weights given in the International Table of Atomic Weights inside
the front cover of the text.

Plan

We add the atomic weights of the elements in the formula, each multiplied by the number of
times the element occurs.

Solution

Number of Atoms of Stated Kind Mass of One Atom Mass Due to Element

2
C 2
12.0 amu  24.0 amu of C

4
H 4
0 1.0 amu  0 4.0 amu of H
2
O 2
16.0 amu  32.0 amu of O

Formula weight (molecular weight) of acetic acid (vinegar) 60.0 amu

You should now work Exercise 26.

2-7

A space-filling model of an acetic

acid (vinegar) molecule,

CH 3 COOH.

2-7 Formula Weights, Molecular Weights, and Moles 63