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Home » Class 11 » Chemistry » Some Basic Concepts of Chemistry » Empirical and Molecular Formula

Empirical and Molecular Formula

Last Updated on January 16, 2023 By Mrs Shilpi Nagpal

Contents

  • 1 Calculation of Percentage Composition
  • 2 Empirical and Molecular Formula 
  • 3 Calculation of Empirical Formula
  • 4 Calculation of 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 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 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.

calculation of n

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.

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.

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 

ElementSymbolPercentage of ElementAtomic
Mass
Moles of ElementSimplest Molar RatioSimplest whole no. Molar ratio
CarbonC24.27122.0211
HydrogenH4.0714.0722
ChlorineCl71.6535.52.0221

The empirical formula of the compound is CH2ClEmpirical mass of CH2Cl = 12 + 2 × 1 +35.5 = 49.5

n=2

Molecular formula = n × E. F.

= 2 × CH2Cl

=C2H4Cl2

Filed Under: Chemistry, Class 11, Some Basic Concepts of Chemistry Tagged With: calculation of percentage composition, empirical formula, molecular formula

About Mrs Shilpi Nagpal

Author of this website, Mrs. Shilpi Nagpal is MSc (Hons, Chemistry) and BSc (Hons, Chemistry) from Delhi University, B.Ed. (I. P. University) and has many years of experience in teaching. She has started this educational website with the mindset of spreading free education to everyone.

Reader Interactions

Comments

  1. kanhiya gupta says

    June 20, 2017 at 9:04 am

    Nice

  2. kanhiya gupta says

    June 20, 2017 at 9:05 am

    Thanks

  3. Sidra says

    July 21, 2018 at 11:34 pm

    Really nice

  4. Keshav kashyap says

    December 28, 2019 at 7:26 pm

    Thanks for this data . I was finding everywhere but could not found but here I got the details …… Thanks ..

  5. Shiwangi says

    July 13, 2020 at 11:56 am

    Good explanation

  6. sherly says

    July 27, 2020 at 9:44 pm

    thank you

  7. Darshini.s says

    September 29, 2020 at 7:07 pm

    Really super it was so helpful. Thank u so much

  8. Ayush Gupta says

    May 11, 2021 at 12:52 pm

    complete and detailed explanation

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