# Molecular Formula Introductory Chemistry Tutorial

## Key Concepts

• A molecular formula tells us how many atoms of each element make up the molecule.(1)
• The atoms of the elements in the formula are represented by their chemical symbols which can be found in the Periodic Table of the Elements. The names of some elements and their corresponding symbols are given below:

 Name of Element Symbol of Element carbon hydrogen oxygen nitrogen C H O N

• The number of atoms of each element is represented by a subscript number to the right of the chemical symbol for the element.
• For the general molecular formula XaYb :
This molecule is made up of two different elements with the chemical symbols X and Y
a and b are whole numbers are greater than 1 ("counting numbers" greater than 1)
This molecule contains a atoms of element X and b atoms of element Y

• If only one atom of a particular element is present, no number is included to the right of the symbol:
For the general molecular formula XYb :
This molecule is made up of two different elements with the chemical symbols X and Y
This molecule contains only 1 atom of element X and b atoms of element Y
• The symbol of an element always begins with a capital letter, so if two capital letters are not separated by a number, the number must be a 1:
We do write XY       We do NOT write X1Y1
We do write X2Y     We do NOT write X2Y1
We do write XY2     We do NOT write X1Y2
• The molecular formula does NOT tell us how the atoms are arranged within the molecule.(2)

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## Using a Molecular Formula to Determine Which Elements are Present in a Molecule

Step 1: Write the molecular formula for the molecule: XaYb :

a and b are both numbers

X and Y are both letters

Step 2: Separate the chemical symbols for each element in the molecular formula by substituting the word "and" for each subscript number:

 X a Y b X and Y and

remove the last "and": X and Y

Step 3: Use the periodic table to find the names of the elements corresponding to the symbols X and Y

### Worked Example of Determining the Elements Present in a Molecule Using Its Molecular Formula

Question 1: A molecule has the molecular formula C2H6. Which elements are present in this molecule?

Solution:

Step 1: Write the molecular formula for the molecule: C2H6

Step 2: Separate the chemical symbols for each element in the molecular formula by substituting the word "and" for each subscript number:

 C 2 H 6 C and H and

remove the last "and": C and H

Step 3: Use the periodic table to find the names of the elements corresponding to the symbols C and H:

C is the symbol for carbon

H is the symbol for hydrogen

C2H6 contains the elements carbon and hydrogen.

Question 2: A molecule has the molecular formula C2H6O. Which elements are present in this molecule?

Solution:

Step 1: Write the molecular formula for the molecule: C2H6O

Step 2: Separate the chemical symbols for each element in the molecular formula by substituting the word "and" for each subscript number:

 C 2 H 6 O C and H and O

Step 3: Use the periodic table to find the names of the elements corresponding to the symbols C and H and O:

C is the symbol for carbon

H is the symbol for hydrogen

O is the symbol for oxygen

C2H6O contains the elements carbon, hydrogen and oxygen.

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## Using a Molecular Formula to Determine the Number of Atoms in a Molecule

If a molecule is made up of just two different elements with the chemical symbols X and Y, then we can write three possible molecular formulae to represent the molecule as shown in the table below:

molecular formula XaYb molecular formula XaY molecular formula XYb
Two elements present:

• Element X, symbol X
• Element Y, symbol Y

This molecule contains:

• a atoms of element X
• b atoms of element Y
Two elements present:

• Element X, symbol X
• Element Y, symbol Y

This molecule contains:

• a atoms of element X
• 1 atom of element Y
Two elements present:

• Element X, symbol X
• Element Y, symbol Y

This molecule contains:

• 1 atom of element X
• b atoms of element Y

Step 1: Write the molecular formula for the molecule :

XaYb

Step 2: Identify the symbols for each element present:

X   a   Y   b

Step 3: Rewrite the molecular formula as a list:

• X   a
• Y   b

Step 4: Rewrite the list so that each subscript number becomes a number to multiply the symbol by:

• X multiplied by a
or, a × X which we read as a atoms of X
• Y multiplied by b
or, b × Y which we read as b atoms of Y

Step 5: Use the periodic table to find the name of each element.

### Worked Example of Determining the Number of Atoms of an Element in a Molecule

Question: A molecule has the molecular formula C2H4O2, how many atoms of hydrogen are present in this molecule?

Solution:

Step 1: Write the molecular formula for the molecule :

C2H4O2

Step 2: Identify the symbols for each element present:

C   2   H   4   O   2

Step 3: Rewrite the molecular formula as a list:

• C   2
• H   4
• O   2

Step 4: Rewrite the list so that each subscript number becomes a number to multiply the symbol by:

• C multiplied by 2
or, 2 × C which we read as 2 atoms of C
• H multiplied by 4
or, 4 × H which we read as 4 atoms of H
• O multiplied by 2
or, 2 × O which we read as 2 atoms of O

Step 5: Use the periodic table to find the name of each element:

C is the symbol for carbon

H is the symbol for hydrogen

O is the symbol for oxygen

So this molecule contains:

2 atoms of carbon

4 atoms of hydrogen

2 atoms of oxygen

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## How to Write the Molecular Formula of a Molecule

If we know how many atoms of each element make up a molecule, then we can write the molecular formula for the molecule.(3)

Step 1: Write a list of the elements present.

Step 2: To the right of each element, write how many atoms of this element are present are in the molecule.

Step 3: Use the periodic table to find the symbol for each element
(if you have been given the names of the elements)

Step 4: Convert each number to a subscript number
(If the number is a one, do not write 1)

Step 5: Rewrite the list on one line as a molecular formula.

### Worked Examples of How to Write a Molecular Formula

Question 1: A molecule is known to contain one atom of carbon and two atoms of oxygen. Write the molecular formula for this molecule.

Step 1: Write a list of the elements present.

Name of Elements Present
carbon
oxygen

Step 2: To the right of each element, write how many atoms of this element are present are in the molecule.

Name of Elements Present Number of Atoms Present
carbon 1
oxygen 2

Step 3: Use the periodic table to find the symbol for each element
(if you have been given the names of the elements)

Name of Elements Present Number of Atoms Present Symbols of Elements
carbon 1 C
oxygen 2 O

Step 4: Convert each number to a subscript number

Name of Elements Present Number of Atoms Present Symbols of Elements Subscript Number
(if number = 1, no number included)
carbon 1 C
oxygen 2 O 2

Step 5: Rewrite the list on one line as a molecular formula.

CO2 is the molecular formula

Question 2: A molecule is known to contain two atoms of carbon, five atoms of hydrogen and one atom of chlorine. Write the molecular formula for this molecule.

Solution:

Step 1: Write a list of the elements present.

Name of Elements Present
carbon
hydrogen
chlorine

Step 2: To the right of each element, write how many atoms of this element are present are in the molecule.

Name of Elements Present Number of Atoms Present
carbon 2
hydrogen 5
chlorine 1

Step 3: Use the periodic table to find the symbol for each element
(if you have been given the names of the elements)

Name of Elements Present Number of Atoms Present Symbols of Elements
carbon 2 C
hydrogen 5 H
chlorine 1 Cl

Step 4: Convert each number to a subscript number

Name of Elements Present Number of Atoms Present Symbols of Elements Subscript Number
(if number = 1, no number included)
carbon 2 C   2
hydrogen 5 H   5
chlorine 1 Cl

Step 5: Rewrite the list on one line as a molecular formula.

C2H5Cl is the molecular formula.

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Footnotes:

(1) A molecule is formed when 2 or more atoms are joined together by covalent bonds. A molecule is a discrete unit. Strictly speaking we should not use the term "molecular formula" to refer to the formula of an ionic substance such as salt because these substances are not discrete and extend outwards in all 3 dimensions, positive ions and negative ions being held together in the lattice by electrostatic forces.
For salts, the chemical formula represents the ratio of cations (positively charged ions) to anions (negatively charged ions), so it is in effect an empirical formula.

(2) How atoms are arranged in a molecule is referred to as its structure. There are different ways to represent the structure of a molecule, one common method is to write a 2-dimensional structural formula.

(3) For the purposes of this tutorial we will assume that the elements listed in the question are in the same order as the order of the symbols in the molecular formula.
If you are interested to know how we decide which symbols to put first and last in the molecular formula you could read AUS-e-TUTE's introductory tutorial on molecular formula of inorganic non-metallic binary compounds.
The molecular formula of inorganic acids and transition metal complexes is a bit different.