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Empirical Formula (OCR GCSE Chemistry A (Gateway))

Revision Note

Alexandra Brennan

Last updated

Empirical Formula

  • Empirical formula is the simplest whole number ratio of atoms of each element in a molecule 

  • The molecular formula tells you the actual number of atoms of each element in a molecule

  • From the molecular formula, we can deduce the empirical formula 

  • The molecular formula will either be given to you or can be worked out from a picture 

  • Sometimes, the empirical formula and molecular formula are the same e.g. CO2

Worked Example

The molecular formula of hexane is C6H14

What is the empirical formula of hexane?

Answer:

  • Divide the number of atoms for each element by a common factor 

  • In this case, both can be divided by 2 

  • The empirical formula is C3H7

Worked Example

What is the empirical formula of the compound below?

An Introduction to AS Level Organic Chemistry Step 1 Deducing isomers of C4H10, downloadable AS & A Level Chemistry revision notes

Answer: 

  • Count the number of atoms of each element: C= 4 H= 10

  • Divide both numbers by a common factor, in this case 2 

  • The empirical formula is C2H5

Empirical Formula of Ionic Compounds

  • If it is a dot-and-cross diagram, then just count the number of atoms of each element

  • This is then equal to the empirical formula of the compound

  • If it is a 3D lattice structure, identify the ions in the lattice

  • Write them down and balance the charges so that the overall charge is zero 

  • The formula of the compound is the empirical formula 

Worked Example

What is the the empirical formula of the ionic compound shown below?

Ionic lattice structures, IGCSE & GCSE Chemistry revision notes

Answer:      

  • Identify the ions in the compound, in this case Na+ and Cl- 

  • Balance the charges so that the overall charge is zero

  • Only one chloride ion is needed to balance out the single positive charge of the sodium ion

  • The empirical formula is NaCl 

Percentage Composition

  • The percentage by mass of an element in a compound can be calculated using the following equation:

percent sign space mass space of space an space element equals fraction numerator A subscript straight r cross times number space of space atoms space of space the space element over denominator M subscript straight r space of space the space compound end fraction cross times 100

Worked Example

Calculate the percentage by mass of calcium in calcium carbonate, CaCO3

Answer

Calculate % Mass of an Element in a Compound WE, downloadable IGCSE & GCSE Chemistry revision notes

Examiner Tips and Tricks

Don’t forget to multiply your answer by 100 in order to convert it to a percentage.

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    Alexandra Brennan

    Author: Alexandra Brennan

    Expertise: Chemistry

    Alex studied Biochemistry at Newcastle University before embarking upon a career in teaching. With nearly 10 years of teaching experience, Alex has had several roles including Chemistry/Science Teacher, Head of Science and Examiner for AQA and Edexcel. Alex’s passion for creating engaging content that enables students to succeed in exams drove her to pursue a career outside of the classroom at SME.