Question

c. What is the most important weak acid in water?

Answer 1

Weak Acids : A weak acid is one which does not ionise fully when it is dissolved in water. Where as strong acids and bases are completely ionized when dissolve in water.

Ethanoic acid and hydrogen sulphide are typical weak acid. It reacts with water to produce hydronium ions and ethanoate ions, but the back reaction is more successful than the forward one. The ions react very easily to reform the acid and the water. The dissociation of ethanoic acid in water as follows:

CH₃COOH +H₂O <=====> CH₃COO- +H₃O+

Most organic acids are weak. Hydrogen fluoride (dissolving in water to produce hydrofluoric acid) is a weak inorganic acid that you may come across elsewhere.

At equilibrium for the dissociation constant (ka) of ethanoic acid is properly written as

ka=[CH3COO-][H3O+] /[CH3COOH]

the Ka values for some weak acids as follows:

acid Ka (mol dm-3)

hydrofluoric acid 5.6 x 10-4

methanoic acid 1.6 x 10-4

ethanoic acid 1.7 x 10-5

hydrogen sulphide 8.9 x 10-8

These are all weak acids because the values for Ka are very small. They are listed in order of decreasing acid strength - the Ka values get smaller as you go down the table.

Hence ethanoic acid and hydrogen sulphide are most weak aacid in water.