The Foundations of Chemistry

For many compounds the molecular formula is a multiple of the simplest formula.
Consider butane, C 4 H 10. The simplest formula for butane is C 2 H 5 , but the molecular for-
mula contains twice as many atoms; that is, 2
(C 2 H 5 )C 4 H 10. Benzene, C 6 H 6 , is an-
other example. The simplest formula for benzene is CH, but the molecular formula con-
tains six times as many atoms; that is, 6
(CH)C 6 H 6.
The molecular formula for a compound is either the same as, or an integermultiple of,
the simplest formula.

Molecular formulan
simplest formula

So we can write

Molecular weightn
simplest formula weight

n

The molecular formula is then obtained by multiplying the simplest formula by the inte-
ger, n.

EXAMPLE 2-16 Molecular Formula

In Example 2-15, we found the elemental composition of glucose. Other experiments show
that its molecular weight is approximately 180 amu. Determine the simplest formula and the
molecular formula of glucose.

Plan

Step 1: We first use the masses of C, H, and O found in Example 2-15 to determine the sim-
plest formula.

Step 2: We can use the simplest formula to calculate the simplest formula weight. Because the
molecular weight of glucose is known (approximately 180 amu), we can determine the molec-
ular formula by dividing the molecular weight by the simplest formula weight.

n

The molecular weight is ntimes the simplest formula weight, so the molecular formula of glu-
cose is ntimes the simplest formula.

Solution

Step 1:

molecular weight



simplest formula weight

molecular weight



simplest formula weight

As an alternative, we could have used

the percentages by mass from Example

2-15. Using the earliest available

numbers helps to minimize the effects

of rounding-off errors.

2-10 Determination of Molecular Formulas 75

Mass of Moles of Element Divide by Smallest Whole-Number
Element Element (divide mass by AW) Smallest Ratio of Atoms

C 0.04055 g 0.003376 mol 1.00 C

H 0.00681 g 0.00676 mol 2.00 H CH 2 O

O 0.0540 g 0.00338 mol 1.00 O

0.00338



0.003376

0.0540



16.00

0.00676



0.003376

0.00681



1.008

0.003376



0.003376

0.04055



12.01