EMPRICAL FIRMULA, MOLECULAR FORMULA, CALCULATIONS INVOLVED TO FIND OUT EMPRICAL AND MOLECULAR FORMULA BY USING COMBUSTION DATA OR COMPOSITION BY MASS

 EMPRICAL FIRMULA:

                       

            The formula which shows the simplest whole number ratio of the elements present in one molecule of a compound is called empirical formula. For example,

            Benzene = CH                                    Glucose = CH²O

Ethane = CH³                         Ethane = CH²

Ethanoic acid = CH²O            Propene = Ch²

Water – H²O

 

MOLECULAR FORMULA:

            The formula which shows the exact (or total) number of atoms of different elements present in one molecule of a compound is called molecular formula.

Compound	Molecular Formula	Empirical Formula
Benzene	C6H6	CH
Butane	C4H10	C2H5
Glucose	C6H12O6	CH2O
Ethanoic Acid	CH3CO2H	CH2O
Water	H2O	H2O
Methane	CH4	CH4

            For many simple compounds the empirical formula and the molecular formula are same. Sometimes empirical formula for different compounds is same but their molecular formula is definitely different from each other.

 

CALCULATIONS INVOLVED TO FIND OUT EMPRICAL AND MOLECULAR FORMULA BY USING COMBUSTION DATA OR COMPOSITION BY MASS:

 

PROBLEM:

            An organic compound is analyzed. It is found  to contain 85.72% carbon and 14.25% hydrogen. Calculate its empirical formula. The molecular mass (Mr) of the compound is 28, and then calculate its molecular formula.

 

ANSWER:

            Empirical formula = CH2

            Molecular formula = C2H4

 

PROBLEM:

A cfc COMPOUND “X” contains the following elements with given % by mass;

            Carbon = 17.8%                    Hydrogen = 1.5%

            Chlorine = 52.6%                  Fluorine = 28.1%

 

            The mass spectrum of “x” shows that the peak with the highest m/e value is at 138. By using the above values, calculate its empirical and molecular formula.

 

ANSWER:

            Empirical formula = CHCIF

            Molecular formula = C₂H₂Cl₂F₂