Empirical & Molecular Formulae (Edexcel IGCSE Chemistry: Double Science)

Revision Note

Test yourself
Stewart

Author

Stewart

Last updated

Did this video help you?

Empirical & Molecular Formulae

  • The molecular formula is the formula that shows the number and type of each atom in a molecule
    • E.g. the molecular formula of ethanoic acid is C2H4O2

  • The empirical formula is the simplest whole number ratio of the atoms of each element present in one molecule or formula unit of the compound
    • E.g. the empirical formula of ethanoic acid is CH2O

  • Organic molecules often have different empirical and molecular formulae
  • The formula of an ionic compound is always an empirical formula

Calculating empirical and molecular formula

How to calculate empirical formulae

  • Empirical formula calculations are very methodical
  • Use a table and the following steps to complete an empirical formula calculation:
    1. Write the element
    2. Write the value given for each element
      • This may be given as a mass, in g, or as a percentage
      • There are exam questions where you are required to calculate the value of one of the elements
    3. Write the relative atomic mass of each element
    4. Calculate the moles of each element
      • Moles = begin mathsize 14px style mass over A subscript r end style
    5. Calculate the ratio of elements
      • Divide all the moles by the smallest number of moles
      • If you get a ratio that does not have whole numbers, you multiply by an appropriate number to make all the values into whole numbers
    6. Write the final empirical formula

Worked example

A sample of a compound was found to contain 10 g of hydrogen and 80 g of oxygen.

Calculate the empirical formula of this compound.

Ar (H) = 1   Ar (O) = 16

Answer:

1. Element H O
2. Value 10 80
3. Relative atomic mass 1 16
4. Moles = begin mathsize 14px style mass over A subscript r end style begin mathsize 14px style 10 over 1 end style = 10 80 over 16 = 5
5. Ratio (divide by smallest) 10 over 5 = 2 5 over 5 = 1
6. Answer The empirical formula is H2O

Worked example

Carbohydrate X was analysed and found to contain 31.58% carbon and 5.26% hydrogen by mass.

Find the empirical formula of carbohydrate X.

Ar (H) = 1   Ar (C) = 12   Ar (O) = 16

Answer:

A carbohydrate contains carbon, hydrogen and oxygen

The percentages do not add up to 100%, which means that you need to calculate the percentage of oxygen needs to be calculated

Percentage of oxygen = 100 - 31.58 - 5.26 = 63.16%

1. Element C H O
2. Value 31.58 5.26 63.16
3. Relative atomic mass 12 1 16
4. Moles = begin mathsize 14px style mass over A subscript r end style begin mathsize 14px style fraction numerator 31.58 over denominator 12 end fraction end style = 2.63 begin mathsize 14px style fraction numerator 5.26 over denominator 1 end fraction end style = 5.26 begin mathsize 14px style fraction numerator 63.16 over denominator 16 end fraction end style = 3.95
5. Ratio (divide by smallest) begin mathsize 14px style fraction numerator 2.63 over denominator 2.63 end fraction end style = 1 fraction numerator 5.26 over denominator 2.63 end fraction = 2 fraction numerator 3.95 over denominator 2.63 end fraction = 1.5
5. Whole number ratio 1 x 2 = 2 2 x 2 = 4 1.5 x 2 = 3
6. Answer The empirical formula is C2H4O3

Examiner Tip

The molar ratio must be a whole number.

If you don't get a whole number when calculating the ratio of atoms in an empirical formula, such as 1.5, multiply that and the other ratios to achieve whole numbers.

How to calculate molecular formula

  • Molecular formula gives the actual numbers of atoms of each element present in the formula of the compound

Table showing the relationship between empirical and molecular formulae

Compound Empirical formula Molecular formula
Methane CH4 CH4
Ethane CH3 C2H6
Ethene CH2 C2H4
Benzene CH C6H6

  

  • To calculate the molecular formula:
    1. Find the relative formula mass of the empirical formula
      • Add the relative atomic masses of all the atoms in the empirical formula
    2. Use the following equation:
      • fraction numerator relative space formula space mass space of space molecular space formula over denominator relative space formula space mass space of space empirical space formula end fraction
    3. Multiply the number of each element present in the empirical formula by the number from step 2 to find the molecular formula

Worked example

The empirical formula of X is C4H10S1

The relative formula mass (Mr ) of X is 180.

Calculate the molecular formula of X.

Ar (C) = 12      Ar (H) = 1      Ar (S) = 32

Answer:

  1. Calculate the relative formula mass of the empirical formula:
    • Mr =  (12 x 4) + (1 x 10) + (32 x 1)   =   90
  2. Divide the relative formula mass of X by the relative formula mass of empirical formula:
    • 180 / 90 = 2
  3. Multiply for the molecular formula:
    • The number of atoms of each elements should be multiplied by 2
    • (C4 x 2) + (H10 x 2) + (S1 x 2)
    • Molecular formula of X = C8H20S2

Deducing formulae of hydrated salts

  • A hydrated salt is a crystallised salt that contains water molecules as part of its structure
  • The formula of a hydrated salt shows the water molecules, e.g. CuSO4•2H2O
    • The • symbol shows that the water present is water of crystallisation
  • The formula of hydrated salts can be determined experimentally by:
    • Weighing a sample of the hydrated salt
    • Heating it until the water of crystallisation has been driven off
      • This is achieved by heating until a constant mass
    • Re-weighing the anhydrous salt
  • From the results, you can determine the mass of anhydrous salt and the mass of the water of crystallisation
  • Applying a similar approach to deducing empirical formulae, the formula of the hydrated salt can be calculated

How to calculate water of crystallisation

  • The steps for empirical formula can be adapted for hydrated salt / water of crystallisation calculations 
    • Instead of writing elements, write the two components of a hydrated salt
      • The salt
      • Water
    • Instead of writing relative atomic mass, write the relative molecular / formula mass of the salt and water
  • Use a table and the following steps to complete the calculation:
    1. Write the salt and water
    2. Write the value given for the salt and water 
      • There are exam questions where you are required to calculate one of these values
    3. Write the relative molecular / formula mass of the salt and water
    4. Calculate the moles of the salt and water
      • Moles = begin mathsize 14px style mass over M subscript r end style
    5. Calculate the ratio salt : water 
      • Divide all the moles by the smallest number of moles
      • The calculation should give a ratio of 1 salt : x water
    6. Write the final hydrated salt formula

Worked example

11.25 g of hydrated copper sulfate, CuSO4.xH2O, is heated until it loses all of its water of crystallisation.

It is re-weighed and its mass is 7.19 g.

Calculate the formula of the hydrated copper(II) sulfate.

Ar (Cu) = 63.5      Ar (S) = 32      Ar (O) = 16     Ar (H) = 1

Answer:

1. Salt and water CuSO4 H2O
2. Value 7.19 11.25 - 7.19
= 4.06
3. Mr  63.5 + 32 + (16 x 4)
= 159.5
(1 x 2) + 16
= 18
4. Moles = begin mathsize 14px style mass over M subscript r end style begin mathsize 14px style fraction numerator 7.19 over denominator 159.5 end fraction end style = 0.045 begin mathsize 14px style fraction numerator 4.06 over denominator 18 end fraction end style = 0.226
5. Salt : water ratio fraction numerator 0.045 over denominator 0.045 end fraction = 1 fraction numerator 0.226 over denominator 0.045 end fraction = 5
6. Formula of hydrated salt The formula is CuSO45H2O  

Examiner Tip

The specification is not clear about whether deducing the formula of hydrated salts is required.

However, it is an application of deducing empirical formulae so it is worth knowing how to do this.

You've read 0 of your 10 free revision notes

Unlock more, it's free!

Join the 100,000+ Students that ❤️ Save My Exams

the (exam) results speak for themselves:

Did this page help you?

Stewart

Author: Stewart

Expertise: Chemistry Lead

Stewart has been an enthusiastic GCSE, IGCSE, A Level and IB teacher for more than 30 years in the UK as well as overseas, and has also been an examiner for IB and A Level. As a long-standing Head of Science, Stewart brings a wealth of experience to creating Exam Questions and revision materials for Save My Exams. Stewart specialises in Chemistry, but has also taught Physics and Environmental Systems and Societies.