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ANALYSIS OF A COMPOUND (EMPIRICAL AND MOLECULAR FORMULAS)

ANALYSIS OF A COMPOUND (EMPIRICAL AND MOLECULAR FORMULAS):
A technique which is used to determine the unknown elements and their proportions in the given sample of a compound is called chemical analysis.
Analysis is of two types:
(i) Qualitative analysis
(ii) Quantitative analysis
(i)  Qualitative analysis:
A technique which is used to determine the unknown elements present in the sample of a compound is called qualitative analysis.
Examples: Salt analysis.
(ii) Quantitative analysis:
A technique which is used to determine mass of each element in the sample of a compound is called quantitative analysis.
Examples: Combustion analysis.
PERCENTAGE OF AN ELEMENT 
Number of grams of an element in 100g of a compound is called percentage of that element.
Percentage of an element   = × 100
EMPIRICAL FORMULA 
A formula which shows the simplest whole number ratio between the atoms present in a compound is called empirical formula.
Examples: H2O for water, CH for benzene (C6H6).
Steps to determine empirical formula:
Empirical formula of a compound can be calculated by the following the steps:
(i)   Determination of the percentage composition of each element.
(ii)  Finding the number of gram atoms (mole) of each element. For this purpose divide the %age of an element by its relative atomic mass.
(iii) Determination of the atomic ratio of each element by dividing gram atoms or mole by the smallest number of moles.
(iv)  If the atomic ratio is in simple whole number, it gives the empirical formula, otherwise multiply with a suitable digit to get the whole number atomic ratio.
COMBUSTION ANALYSIS 
The analysis made to determine the percentage composition of an organic compound by burning a known quantity of it in excess of oxygen is called combustion analysis.
  • This analysis is used to analyze those organic compounds which contain C, H and O only.
  • The sole products will be CO2and H2O. These two products of combustion are separately collected.
Calculations:
Following formula are used to get the percentage of carbon, hydrogen and oxygen, respectively.
% of Hydrogen   
% of Carbon     
The percentage of oxygen is obtained by the method of difference.
% of Oxygen    = 100 - (% of C + % of H)

The formula of a substance which is based upon the actual ratio between the atoms in a molecule of a compound is molecular formula.
Examples: C6H6 and C6H12O6 are the molecular formulas of benzene and glucose.
Relationship between empirical and molecular formula:
Molecular formula = n x Empirical formula
  • Molecular formula is integral multiple of empirical formula.
  • Where, ‘n’ is a simple integer.
  • The value of n is the ratio of molecular mass and empirical formula mass.
n = 
(i) The value of n is greater than one for all those compounds which have different empirical formula and molecular formula. Molecular formula is simple multiple of empirical formula. e.g. n = 6 and n = 2 for benzene and acetic acid respectively.
(ii) The value of n is unity for all those compounds which have same empirical and molecular formula. e.g. n = 1 for water (H2O) and sucrose (C12H22O11).

Difference between molecular formula and empirical formula 

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