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The carboxylic acids form a homologous series.

The general formula for carboxylic acid is C_nH_2nO₂. The molecular formula is usually written with COOH functional group.

Example

Write the molecular formula of the carboxylic acid with 4 carbon atom.

General formula is C_nH_2nO₂

n = 4

2n = 8

C₄H₈O₂

removing COOH leaves C₃H₇ so the molecular formula may be written C₃H₇COOH.

The functional group in the carboxylic acids is the carboxyl group –COOH. It is responsible for the carboxylic acids’ typical reactions. They are weak acids – vinegar is a dilute solution of ethanoic acid.

The first four carboxylic acids - higher tier

Acid properties

The carboxylic acids have properties typical of acids. They:

• dissolve in water to form acidic solutions with pH values less than 7
• react with metals to form a salt and hydrogen
• react with bases to form a salt and water
• react with carbonates to form a salt, water and carbon dioxide

These properties are due to the –COOH functional group.

Weak and strong acids (Higher tier only)

Carboxylic acids are weak acids because they only partially ionise in solution.

Their solutions do not contain many hydrogen ions compared to a solution of a strong acid at the same concentration. A weak acid’s pH will be higher than a strong acid’s pH at the same concentration. In a solution of strong acid, the molecules are fully ionised. In a weak acid, few of the molecules are ionised.