1.1 What is Chemistry?

http://www.ck12.org Chapter 10. The Mole

A compound of iron and oxygen is analyzed and found to contain 69.94% iron and 30.06% oxygen by mass. Find
the empirical formula of the compound.

Answer:

Follow the steps outlined in the text.

1. Assume a 100 g sample. In 100 grams of the compound, there would be 69.94 g Fe and 30.06 g O.


2. Convert to moles.

69 .94 g Fe×

1 mol Fe

55 .85 g Fe

= 1 .252 mol Fe

30 .06 g O×

1 mol O

16 .00 g O

= 1 .879 mol O

3. Divide both values by the smallest of the results.

1 .252 mol Fe

1. 252

=1 mol Fe

1 .879 mol O

1. 252

= 1 .501 mol O

4. Since the moles of O is still not a whole number, both numbers can be multiplied by 2. The results are now close
   enough to be rounded to the nearest whole number.

1 mol Fe× 2 =2 mol Fe

1 .501 mol O× 2 =3 mol O

The empirical formula of the compound is Fe 2 O 3.

Molecular Formulas

Molecular formulastell us how many atoms of each element are present in one molecule of a molecular compound.
In many cases, the molecular formula is the same as the empirical formula. For example, the molecular formula of
methane is CH 4 , and because 1:4 is the smallest whole-number ratio that can be written for this compound, that is
also its empirical formula. Sometimes, however, the molecular formula is a simple whole-number multiple of the
empirical formula. Acetic acid is an organic acid that gives vinegar its distinctive taste and smell. Its molecular
formula is C 2 H 4 O 2. Glucose is a simple sugar that cells use as their primary source of energy. Its molecular formula
is C 6 H 12 O 6. The structures of both molecules are shown inFigure10.6. They are very different compounds, yet
both have the same empirical formula, CH 2 O.

The followingTable10.1 shows a few other compounds with their empirical and molecular formulas:

TABLE10.1:Empirical and Molecular Formulas

Compound Empirical Formula Molecular Formula

water H 2 O H 2 O

hydrogen peroxide HO H 2 O 2

methane CH 4 CH 4

butane C 2 H 5 C 4 H 10 239