Carboxylic Acids (Edexcel IGCSE Chemistry): Revision Note

Careful: When using the general formula C_nH_2n+1COOH, the 'n' does not represent the total number of carbons in the molecule.

It only counts the carbons in the alkyl chain.

This is a common point of confusion. For example, methanoic acid (HCOOH), n = 0!

Use the general formula to find the molecular formula for propanoic acid.

Answer:

• The name "prop-" means the molecule has 3 carbon atoms in total

• One carbon is part of the -COOH functional group

• Therefore, the number of carbons in the alkyl group is 3 - 1 = 2

  • So, n = 2

• Substitute n=2 into the general formula C_nH_2n+1COOH:

  • C₂H_{(2 x 2) + 1}COOH simplifies to C₂H₅COOH

• This gives the final molecular formula of C₃H₆O₂

Use the general formula to find the molecular formula for pentanoic acid.

Answer:

• The name "pent-" means the molecule has 5 carbon atoms in total

• One carbon is part of the -COOH functional group

• Therefore, the number of carbons in the alkyl group is 5 - 1 = 4

  • So, n = 4

• Substitute n=4 into the general formula C_nH_2n+1COOH:

  • C₄H_{(2 x 4) + 1}COOH simplifies to C₄H₉COOH

• This gives the final molecular formula of C₅H₁₀O₂