Page 69 - Chemistry_Form_2

Chemistry
for Secondary Schools

Types of chemical formulas
Chemical formulas can be divided into three types, namely empirical formula,
molecular formula, and structural formula.

An empirical formula is the formula which represents the simplest ratio of the
FOR ONLINE READING ONLY
atoms or ions in a compound. The simplest formula is usually determined by
considering experimental data. That is why it is called ‘empirical’ which means
‘based on experimentation’. For example, CH shows there are twice as many
2
hydrogen atoms as carbon atoms. It does not show the exact number of each atom
of the element in the compound.

A molecular formula shows the actual number of each atom in a molecule. It is
a multiple of the empirical formula. For example, if the empirical formula is CH ,
2
its molecular formula may be C H , C H , C H , and so on. Therefore, a molecular
8
6
12
2
4
4
formula is equal to n multiplied by the empirical formula, where n is a whole
number. Note that when n is 1, the empirical formula equals the molecular formula.
A structural formula is a graphic representation of molecular structure showing
how the atoms are arranged. At this level, only the empirical and molecular
formulas will be studied.
Formula calculations

When the percentage compositions of the elements that make up a compound
are known, it is possible to obtain both the empirical and molecular formulas of
such a compound. The following are the steps considered when calculating the
empirical formula:
Step 1: Obtain the mass of each element in the sample compound. If expressed
in percentages, convert the percentage of each of the elements to mass.
If the mass or relative molecular mass (R.M.M.) of the compound is
not given, an arbitrary mass of 100 g is usually used.

Note: The R.M.M. is the sum of the relative atomic masses (R.A.M.)
of all the atoms in a molecule of the compound. The R.A.M.
can also be obtained from the periodic table (See Appendix 1).
Step 2: Divide the mass of each element by its R.A.M.
Step 3: Divide each of the values obtained in Step 2 by the lowest value
among them.
Step 4: Convert the ratios in Step 3 to whole numbers. These whole numbers
give the ratio of each element in the compound.

62 Student’s Book Form Two

01/08/2025 11:20:56
CHEMISTRY FORM TWO NEW 2025 DUMMY.indd 62 01/08/2025 11:20:56
CHEMISTRY FORM TWO NEW 2025 DUMMY.indd 62