Interpreting chemical formulas

Before we can proceed to balance chemical equations, we must first know how to interpret a chemical formula in terms of the number of atoms of each element.

Example:

How many atoms of each element are in the following chemical formulas?

a) CO₂

b) 2NH₃

c) 2CH₃OH

d) Ba(NO₃)₂

e) 2Al₂(SO₄)₃

Solution:

a) CO₂

• The C indicates that it has one carbon atom.
• The O₂ indicates that it has two oxygen atoms.
• Note that the subscript, in this case 2, is only for the oxygen atom just before it. It does not affect the carbon atom.

b) 2NH₃

• For each NH₃ molecule, the N indicates that it has one nitrogen atom and the H₃ indicates that it has three hydrogen atoms.
• The number 2 in front (called the coefficient) indicates that there are two NH₃ molecules.
• So, in total, there are two nitrogen atoms and six hydrogen atoms.

c) 2CH₃OH

• Each CH₃OH molecule has one carbon atom, four hydrogen atoms and one oxygen atom.
• The coefficient of 3 in front indicates that there are three CH₃OH molecules.
• So, in total, there are three carbon atoms, twelve hydrogen atoms and three oxygen atoms.

d) Ba(NO₃)₂

• If you have a bracket then the subscript must be multiplied into all the atoms in the bracket. (NO₃)₂ would mean that there are two nitrogen atoms and six oxygen atoms.
• So, in total, there are one barium atom, two nitrogen atoms and six oxygen atoms.

e) 2Al₂(SO₄)₃

• (SO₄)₃ would mean that there are three sulfur atoms and twelve oxygen atoms.
• The coefficient of 2 in front indicates that there are two Al₂(SO₄)₃ molecules.
• So, in total, there are four aluminum atoms, six sulfur atoms and 24 oxygen atoms.

Atoms can neither be destroyed nor created. Therefore, in a chemical reaction

The sum of atoms before reaction = the sum of atoms after reaction

Balancing chemical equations may require some trial and error. There are some general rules that could be helpful, but they may not work all the time.

• Rule 1: Balancing chemical equations using the one’s and two’s technique
• Rule 2: Balancing chemical equations using the two’s and three’s technique
• Rule 3: Balancing chemical equations using the CHO technique
• Rule 4: Balancing chemical equations using the even technique
• Rule 5: Balancing chemical equations containing polyatomic ions

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