Molecular Formula

Molecular formula

 

The molecular formula is the expression of the number of atoms of each element in one molecule of a compound.

The molecular formula definition is the formula showing the actual number of each atom in a molecule.

When the molar mass value is known, the Molecular Formula is calculated by the empirical formula.  

 

 

Molecular_formula

 

 

 

The molecular formula is often same as empirical formula or an exact multiple of it.

 

Example 1

Caffeine has the following composition: 49.48% of carbon, 5.19% of hydrogen, 16.48% of oxygen and 28.85% of nitrogen. The molecular weight is 194.19 g/mol. Find out the molecular and empirical formula.

 

Solution

Step 1

Multiply percent composition with the molecular weight

Carbon – 194.19 x 0.4948 = 96.0852

Hydrogen – 194.19 x 0.0519 = 10.07846

Oxygen –  194.19 x 0.1648 = 32.0025

Nitrogen – 194.19 x 0.2885 = 56.0238

 

Step 2

Divide each value by the atomic weight

Carbon : 96.0852 / 12.011 = 7.9997

Hydrogen : 10.07846 / 1.008 = 9.998

Oxygen : 32.0025 / 15.9994 = 2.000

Nitrogen : 56.0238 / 14.0067 = 3.9997

 

Step 3

Round off the values to closest whole number

8: Carbon

10: Hydrogen

2: Oxygen

4: Nitrogen

Hence, the molecular formula is C8H10N4O2.

 

Step 4

Since 2 is the common factor among 8, 10, 4 and 2.

The empirical formula is C4H5N2O.

 

Example 2

A compound contains 86.88% of carbon and 13.12% of hydrogen, and the molecular weight is 345. What will be the empirical and molecular formula of the compound?

 

Solution

Step 1

Carbon : 345 x 0.8688 = 299.736

Hydrogen : 345 x 0.1312 = 45.264

 

Step 2

Carbon : 299.736 / 12.011 = 24.995

Hydrogen : 45.264 / 1.008 = 44.91

 

Step 3

Molecular formula is C25H45

 

Step 4

Since 5 is the common factor, the empirical formula is C5H9

 

To know more examples and practice questions on molecular formulas, please visit Byju’s.com

 


Practise This Question

The ratio of corresponding sides for the pair of triangles whose construction is given as follows :
Triangle ABC of dimesions AB=4cm,BC= 5 cm and ∠B= 60o.
A ray BX is drawn from B making an acute angle with AB.
5 points B1,B2,B3,B4 and B5 are located on the ray such that BB1=B1B2=B2B3=B3B4=B4B5.
B4 is joined to A and a line parallel to B4A is drawn through B5 to intersect the extended line AB at A'.
Another line is drawn through A' parallel to AC, intersecting the extended line BC at C'. Find the ratio of the corresponding sides of ΔABC and ΔABC.