Question

The pH is less than 7 for the solution of:
A. $FeC{l_3}$
B. $NaCN$
C. $NaOH$
D. $NaCl$

Answer 1

To solve this question, we should know about different types of acid base reactions and the pH of the salts formed by their neutralisation reaction. pH of the solution formed after an acid-base reaction is dependent upon the concentration of $[{H^ + }]$ or $[O{H^ - }]$ ions present in the solution, which was not neutralised.

Complete step by step answer:
As we know that the pH scale is used to measure the concentration of $[{H^ + }]$ ions in a solution. A pH less than 7 means the solution is acidic while a pH more than 7 means a basic solution. An ideally neutral solution has a pH of exactly 7.
In Option A, $FeC{l_3}$ is formed by the reaction of a strong acid $HCl$ and weak base $Fe{(OH)_3}$. During the reaction, dissociation of $[{H^ + }]$ions will be more than $[O{H^ - }]$ ions. Thus, after the formation of salt and water, there will still be $[{H^ + }]$ ions in the solution. Thus, the solution of $FeC{l_3}$ will be acidic and have a pH of less than 7. The acid-base reaction will be as follows:
$HCl + Fe{(OH)_3} \to FeC{l_3} + {H_2}O$
In option B, $NaCN$ is formed by the reaction of a weak acid $HCN$ and a strong base $NaOH$. During the reaction, dissociation of $[O{H^ - }]$ ions will be more than $[{H^ + }]$ ions. So, the salt will be formed in a basic solution and thus, will have a pH of more than 7. The acid-base reaction will be as follows:
$HCN + NaOH \to NaCN + {H_2}O$
In option C, $NaOH$ is not a salt, it is a strong base and in a solution will dissociate into $[N{a^ + }]$ and $[O{H^ - }]$ ions. It will have a pH a lot higher than 7. The dissociation of $NaOH$ will be as:
$NaOH \to N{a^ + } + O{H^ - }$
In option D, $NaCl$ is formed by the reaction of a strong acid $HCl$ and a strong base $NaOH$. During the reaction, the dissociation of $[{H^ + }]$ ions and $[O{H^ - }]$ ions will be equal. Thus, the solution formed will be completely neutral and have a pH of 7. The acid-base reaction will be as follows:
$HCl + NaOH \to NaCl + {H_2}O$

$\therefore$ The correct option is option A, i.e. $FeC{l_3}$ .

Note:
Strong acids usually form acidic salts with weak bases and neutral salts with strong bases. This is because strong acids have high concentration of $[{H^ + }]$ ions which are not fully neutralised by weak bases but are neutralised by strong bases. Weak acids usually form basic salts with strong bases and neutral salt with weak bases. This is because weak acids have lower concentration of $[{H^ + }]$ ions which can neutralise $[O{H^ - }]$ in weak bases but fail to neutralise all the $[O{H^ - }]$ ions in strong bases.