Question

Explain the differences observed between a buffer solution and the distilled water with respect to adding the 2 drops of 1 M HCl and 2 drops of NaOH.

Please provide a detailed explanation.

Answer 1

buffer solutions are solutions that resist change in pH upon addition of small amount of acid or base.

There are two kinds of buffers

1.Acidic buffers

2.Alkaline buffers

Acidic buffers

An acidic buffer is a mixture of a weak acid and one of its salts.They have a pH below 7.

Alkaline buffers

An acidic buffer is a mixture of a weak base and one of its salts.They have a pH above 7.

A mixture of acetic acid (CH₃COOH) (weak acid ) and sodium acetate (CH₃COONa) (it's salt) is an acidic buffer .Acetic acid is weakly ionized because weak acids do not ionize completely in water while sodium acetate is almost completely ionized because salts completely ionize in water.

CH₃COOH H⁺ + CH₃COO^–

CH₃COONa Na⁺ + CH₃COC^–

To this, if you add a drop of a strong acid like HCl, the H⁺ ions from HCl combine with CH₃COO^– to give CH₃COOH.Since CH₃COOH is a weak acid it does not ionize completely .Thus, there is a very slight change in the pH value. Now, if you add a drop of NaOH, the OH^– ions react with the free acid to give undissociated water molecules.

CH₃COOH + OH^–   CH₃COO^– + H₂O

In this way, the OH^– ions of NaOH are removed and the pH does not change.

a mixture of ammonium hydroxide ( weak base ) and ammonium chloride ( it's salt ) is a alkaline buffer solution.ammonium hydroxide is weakly ionized because weak bases do not ionize completely in water while ammonium chloride is almost completely ionized because salts completely ionize in water.

NH₄OH NH₄⁺ + OH^–

NH₄Cl NH₄⁺ + Cl^–

To this, if you add a drop of a strong acid like HCl, the H⁺ ions from HCl combine with OH^– to give H₂O.Therefore the pH does not change.

Now, if you add a drop of NaOH, the OH^– ions react with the NH₄⁺ to form NH₄OH. Since NH₄OH is a weak base it does not ionize completely.Thus, there is a very slight change in the pH value

NH₄⁺ + OH^–   NH₄OH

If you add HCl or NaOH to water they dissociate to give H⁺ and OH^- ions and remain in water in that form.Therefore pH immediately changes upon addition.