Question

Choose the correct from the following statements:
(i) Salt of a strong acid and a strong base are neutral with a pH value of 7.
(ii) Salt of strong acid and a weak base are basic with a pH value more than 7.
(iii) Salt of a weak acid and strong base are acidic with a pH value less than 7.
[A] (i) and (ii)
[B] (ii) and (iii)
[C] (i)
[D] (i) and (iii)

Answer 1

Hint: A salt is acidic or basic or neutral depending upon the ions it dissociates in a solvent. If we have a strong acid and a weak base, there are hydrogen ions in excess. If we have a weak acid and a strong base the solution has excess hydroxide ions.

Complete step by step answer:
We know that when we mix an acid with a base we get salt and water. Depending upon the strength of the acid and the base, the obtained salt is either acidic or basic or neutral.
An acidic salt when dissolved in a solute makes the solution slightly acidic whereas when a basic salt is dissolved in a solute, it makes the solution slightly basic. A neutral salt does not change the acidity of the solution and keeps it neutral.
Now, let us discuss each of the options to find out if they are correct or incorrect.
In the first option we have salt of a strong acid and a strong base is neutral with a pH value of 7. Neutralisation of a strong acid by a strong base gives us a salt of neutral pH. On dissolution, it does not change the pH of the solution. For example sodium chloride which is formed by the neutralisation of hydrochloric acid by sodium hydroxide. Sodium chloride in water shows neither acidic nor basic properties.
     \[NaOH+HCl\to NaCl+{{H}_{2}}O\]
Therefore, this statement is correct.

In the next option we have Salt of strong acid and a weak base is basic with a pH value more than 7. A strong acid with a weak base gives us an acidic salt. We know that the pH value of an acid is less than 7. For example ammonium sulphate is formed by the neutralisation of sulphuric acid which is a strong acid by ammonium hydroxide which is a weak base.
     \[{{H}_{2}}S{{O}_{4}}(S.A)+N{{H}_{4}}OH(W.B)\to {{\left( N{{H}_{4}} \right)}_{2}}S{{O}_{4}}+{{H}_{2}}O\]
Therefore, this statement is incorrect.

And lastly we have salt of a weak acid and strong base are acidic with a pH value less than 7.
A strong base with a weak acid will give us a basic salt whose pH value will be more than 7. For example sodium acetate which is formed by a strong base sodium hydroxide and a weak acid acetic acid. Sodium acetate in water makes the solution slightly basic.
     \[NaOH+C{{H}_{3}}COOH\to NaC{{H}_{3}}COO+{{H}_{2}}O\]
Therefore, this statement is also incorrect.

We can understand from the above discussion that only the first statement is correct.
So, the correct answer is “Option C”.

Note: We identify a salt to be acidic, basic or neutral from the ions it dissociates into a polar solvent like water. The salts make the solution acidic or basic due to these corresponding ions in the solution. A solution is acidic if there are more hydrogen ions in the solution than hydroxide ions and the solution is basic if there are more hydroxide ions. However, it does not necessarily have to be hydrogen and hydroxide ions. It can be any proton donor (acid) or proton acceptor (base) ions.