Question

Identify which of the following is not correct for
$ {{K}_{W}}$ of $ {{H}_{2}}O$ at $ 373K$ is $ 1\times {{10}^{-12}}$ ,
(a) ) $ p{{K}_{W}}$ of $ {{H}_{2}}O$ is $ 12$
(b) pH of $ {{H}_{2}}O$ is $ 6$
(c) $ {{H}_{2}}O$ is neutral
(d) $ {{H}_{2}}O$ is acidic

Answer 1

Hint You should properly know about the concept of the pH, dissociation constant of the acid and pKa value of the acid and their formulas and then, by using them you can easily find the correct options and the incorrect option from the above given options. Now solve it.

Complete step by step answer:
First of all, let’s discuss what is pH. pH gives us the measure of acid/base strength of any solution. pH scale ranges from 0-14, 7 is neutral, below 7 it represents acidic nature i.e. the compound is acidic and above 7 it represents basic nature i.e. the compound is basic.
Now, coming next to the $ Ka$ value. The $ pKa$ tells us about the strength of the acid. The $ pKa$ is the negative logarithm of the dissociation constant i.e. $ Ka$ . Smaller value of $ pKa$ indicates that it is a strong acid and it has a large $ Ka$ value. On the other hand, a larger value of $ pKa$ indicates that it is a weak acid and it has a large $ Ka$ value.
We will consider the statement one by one as;
(a) $ p{{K}_{W}}$ of $ {{H}_{2}}O=-\log {{K}_{W}}$
As we know that, the $ p{{K}_{W}}$ value of the water is 12 (given)
Then;
$ p{{K}_{W}}$ of $ {{H}_{2}}O=-\log {{10}^{-12}}=12$
So, the option (a) is correct.

(b) pH of $ {{H}_{2}}O$ is $ 6$
The dissociation of water occurs as;
$ {{H}_{2}}O\rightleftharpoons [{{H}^{+}}][O{{H}^{-}}]$
The dissociation constant of water is;
$ {{K}_{W}}=[{{H}^{+}}][O{{H}^{-}}]$ ---------(1)
As the concentration of both the hydrogen and hydronium ion is same, so $ [{{H}^{+}}]=[O{{H}^{-}}]$
The equation becomes as;
$ {{K}_{W}}=[{{H}^{+}}][{{H}^{+}}]={{[{{H}^{+}}]}^{2}}$ ---------(2)
We know, that the concentration of $ [{{H}^{+}}]={{10}^{-12}}$ (Given)
Then, put this value in equation (2), we get;
$ {{K}_{W}}=\sqrt{{{10}^{-12}}}={{10}^{-6}}$
Then, the pH is;
$ pH=-\log {{10}^{-6}}=6$
So, thus, the option (b) is also correct.

(c) $ {{H}_{2}}O$ is neutral
As both the hydrogen and hydronium ions are the same, so $ [{{H}^{+}}]=[O{{H}^{-}}]$ , so, thus water is neutral.
Hence, option (c) is also correct.

(d) $ {{H}_{2}}O$ is acidic
From the option (c) , we come to know that water is neutral and not acidic. So, thus, this option is incorrect.

Hence, from all the above given options, option (d) is not correct.

Note: Don’t get confused in the $ pKa$ and $ Ka$ .
$ pKa$ value gives us the strength of the acid i.e. it tells whether the given acid is a strong acid or a weak acid.$ pKa$ value is the negative logarithm of the $ Ka$ value i.e. $ pKa=-\log 10\text{ Ka}$ .
On the other hand, $ Ka$ is the dissociation constant of the acid.
Smaller value of $ pKa$ indicates that it is a strong acid and it has a large $ Ka$ value. On the other hand, a larger value of $ pKa$ indicates that it is a weak acid and it has large $ Ka$ value.