Molecular Formula Tutorial

Key Concepts

• A molecular formula tells us how many atoms of each element make up the molecule.

• The atoms of the elements in the formula are represented by their chemical symbols.

 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 elements: X and Y
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 elements: 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 write:
XY NOT X1Y1
X2Y NOT X2Y1
XY2 NOT X1Y2

• The molecular formula does NOT tell us how the atoms are arranged within the molecule.

Determining 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

Example:

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

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.

Determining the Number of Atoms in a Molecule

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 X which we read as a atoms of X

• Y multiplied by b
or, b x Y which we read as b atoms of Y

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

Example:

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

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 x C which we read as 2 atoms of C

• H multiplied by 4
or, 4 x H which we read as 4 atoms of H

• O multiplied by 2
or, 2 x 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

Writing Molecular Formula

If we know how many atoms of each element make up a molecule, then we can write the molecular formula for the 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
(If the number is a one, do not write 1)

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

Example:

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 PresentNumber of Atoms Present
carbon1
oxygen2

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 PresentNumber of Atoms PresentSymbols of Elements
carbon1C
oxygen2O

Step 4: Convert each number to a subscript number

Name of Elements PresentNumber of Atoms PresentSymbols of ElementsSubscript Number
(if number = 1, no number included)
carbon1C
oxygen2O2

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

CO2 is the molecular formula.

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