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:

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

Answer

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 Example : 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 C₆H_{6 ,} hydrogen peroxide is H₂O₂ , Glucose is C₆H_6.

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:

Molecular mass = 2 × Vapour Density

Calculation of Empirical Formula

Step 1 : Convert the mass percentage into grams.

Step 2 : Calculate the number of moles.

Step 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.

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	2	1

n=2

Molecular formula = n × E. F.

= 2 × CH₂Cl

=C₂H₄Cl₂