Question

Which of these salts will give the highest pH in water?
[A] KCl
[B] NaCl
[C] $N{{a}_{2}}C{{O}_{3}}$
[D] $CuS{{O}_{4}}$

Answer 1

Hint: To solve this firstly remember that higher pH means the solution needs to have hydroxyl ions. Write down the dissociation of the given compounds in water and identify the one that will give hydroxyl ions. Also remember that a salt of weak acid and strong base will give a basic solution.

Complete answer:
We know that in chemistry, salt is a chemical compound which contains cation and anions in specific proportion so that it is electrically neutral.
 An acid salt is a salt which will produce an acidic solution after being dissolved in a solvent. We can form then neutralizing strong acids with a weak base.
And a basic salt is a salt which upon dissolving in a solution will form a basic solution. We can form basic salts by neutralising strong bases with a weak acid.
Here, we have to find the salt that will give the highest pH in water. We know that higher the pH, lower is the acidity and higher is the acidity of the solution.
Therefore, the salt that will give the highest pH in water here should be a basic salt. Here, for the solution to have higher hydroxyl ions, it needs to be the salt of a strong base and a weak acid.
Now, let us see the dissociation of the salts given to us.
Firstly we have KCl and it will dissociate as- $KCl\to {{K}^{+}}+C{{l}^{-}}$ . Now, KCl in water will not give us hydroxyl ions thus the solution will not be basic. It is a salt of HCL i.e. a strong acid.
Then we have NaCl. It will dissociate into- $NaCl\to N{{a}^{+}}+C{{l}^{-}}$. Here also, NaCl in water will not give us hydroxyl ions thus the solution will not be basic. It is a salt of HCL i.e. a strong acid.
Next we have $N{{a}_{2}}C{{O}_{3}}$. It will dissociate as- $N{{a}_{2}}C{{O}_{3}}\to 2N{{a}^{+}}+C{{O}_{3}}^{2-}$. In water, it will give us-$\begin{align}
  & C{{O}_{3}}^{2-}+{{H}_{2}}O\rightleftharpoons HC{{O}_{3}}^{-}+O{{H}^{-}} \\
 & N{{a}^{+}}+O{{H}^{-}}\to NaOH \\
 & HC{{O}_{3}}^{-}+{{H}^{+}}\to {{H}_{2}}C{{O}_{3}} \\
\end{align}$
We can see here that it is a salt of a weak acid i.e, carbonic acid and a strong base i.e. NaOH and also in water it dissociates into hydroxyl ions. So, the solution will be basic in this case.
And lastly, we have $CuS{{O}_{4}}$ which will dissociate as $CuS{{O}_{4}}\to C{{u}^{2+}}+S{{O}_{4}}^{2-}$ and here also we will not get a basic solution. It is a salt of sulphuric acid i.e. a strong acid.

Therefore, the correct answer is option [C] $N{{a}_{2}}C{{O}_{3}}$ .

Note:
We identify a salt to be acidic, basic or neutral from the ions it dissociates into in 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.