Everything Science Grade 11

CHAPTER 6. QUANTITATIVE ASPECTS OF CHEMICAL CHANGE 6.5

6.5 The Composition ofSubstances ESBBK

The empirical formula of a chemical compound is a simple expressionof the relative number of each

type of atom in that compound. In contrast, the molecular formula of a chemical compound gives the

actual number of atomsof each element found in a molecule of that compound.

DEFINITION: Empirical formula

The empirical formula of a chemical compoundgives the relative number of each type

of atom in that compound.

DEFINITION: Molecular formula

The molecular formulaof a chemical compound gives the exact number of atoms of

each element in one molecule of that compound.

The compound ethanoic acid for example, has the molecular formula CH 3 COOH or simply C 2 H 4 O 2.

In one molecule of thisacid, there are two carbon atoms, four hydrogenatoms and two oxygen atoms.

The ratio of atoms in thecompound is 2:4:2, which can be simplified to 1:2:1. Therefore, the empirical

formula for this compound is CH 2 O. The empirical formula contains the smallestwhole number ratio

of the elements that make up a compound.

Knowing either the empirical or molecular formula of a compound, canhelp to determine its compo-

sition in more detail. The opposite is also true. Knowing the composition of a substance can helpyou

to determine its formula. There are three different types of composition problems that you mightcome

across:

1. Problems where youwill be given the formula of the substance and asked to calculate the
   percentage by mass of each element in the substance.


2. Problems where youwill be given the percentage composition andasked to calculate the for-
   mula.


3. Problems where youwill be given the products of a chemical reactionand asked to calculate the
   formula of one of the reactants. These are oftenreferred to as combustion analysis problems.

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