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	carbon	hydrogen	oxygen	nitrogen
  Symbol of Element	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 X_aY_b :

  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 XY_b :
  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⁾

Please do not block ads on this website.
No ads = no money for us = no free stuff for you!

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

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

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:

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:

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:

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.

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:

Step 1: Write the molecular formula for the molecule :

X_aY_b

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

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.

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

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.

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

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

C₂H₅Cl is the molecular formula.