Empirical and Molecular Formula

Calculation of percentage composition

The percentage of any element or constituent in a compound is the number of parts by mass of that element or constituent present in 100 parts by mass of the compound.

Step 1 : Calculate the molecular mass of the compound from its formula by adding the atomic masses of the element present.

Step 2 : Calculate the percentage of elements or the constituents by applying the relation:

Percentage composition

Question : Calculate the percentage of Carbon, Hydrogen  and oxygen in Ethanol?

Answer :

Percentage composition in ethanol

Empirical and Molecular Formula 

The empirical formula of a compound is the chemical formula which expresses the simplest whole number ratio of the atoms of the various elements present in one molecule of the compound.

For Ex: The empirical formula of benzene is CH, hydrogen peroxide is HO, Glucose is CH2O.

The empirical formula represents only the atomic ratio of the various elements present in its molecule.

The molecular Formula of a compound is the chemical formula which represents the true formula of its molecules.It expresses the actual number of atoms of various elements present in one molecule of the compound.

The molecular formula of benzene is C6H6 , hydrogen peroxide is H2O2 , Glucose is C6H6.

Molecular Formula = n × Empirical formula

where n is any integer such as 1, 2, 3 ,4 etc.

The value of n can be obtained from the relation:

calculation of n

Molecular mass = 2 × Vapour Density

Calculation of Empirical Formula

Step 1 : Convert the mass percentage into grams.

Step 2 : Calculate the number of moles.

number of molesStep 3 : Calculate the simplest molar ratio: Divide the moles obtained in step 1 by the smallest quotient or the least value from amongst the values obtained for each element.

Step 4 : Calculate the simplest whole number ratio.

Step 5 : Write the empirical formula.

Empirical Formula

The empirical Formula is C2H4O.

Calculation of molecular Formula

The molecular formula of a compound can de deduced from:

1)Empirical Formula

2)Molecular mass

Step 1 Calculation of the empirical formula from the percentage composition.

Step 2 Calculation of empirical formula mass by adding the atomic mass of all the atoms present in the empirical formula.

Step 3 Determination of the molecular mass of the compound from the given data.

Step 4 Determination of the value of n.

Step 5 Determination of the molecular formula

Question  A compound contains 4.07 % hydrogen, 24.27 % carbon and 71.65 % chlorine.Its molar mass is 98.96 g.What are its empirical and molecular formula.

Answer 

Element Symbol Percentage

Of element

Atomic mass Moles of

element

Simplest

Molar ratio

Simplest

Whole no.

Molar ratio

Carbon C 24.27 12 2.02 1 1
Hydrogen H 4.07 1 4.07 2 2
Chlorine Cl 71.65 35.5 2.02 1 1

The empirical formula of the compound is CH2Cl

Empirical mass of CH2Cl = 12 + 2 ×1 +35.5 = 49.5

n=2

Molecular formula = n × E. F.

= 2 × CH2Cl

=C2H4Cl2

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