Question

The weakest acid among the following acid-
A) Peroxyacetic acid
B) Acetic acid
C) Chlororacteic acid
D) Tricholoacetic acid
E) Propanoic acid.

Answer 1

Consider the ${{P}_{{{K}_{a}}}}$ value of the acids.
The less the ${{P}_{{{K}_{a}}}}$ value, the acid will be a strong acid.

Complete Solution :
So in the question five acids are given and we have to choose which is the least acidic among them.
If we look at the options we know that all the acids are weak acids so it is always convenient for us to take the ${{P}_{{{K}_{a}}}}$ value for the weak acids.
- Now let’s see a basic idea about what is ${{P}_{{{K}_{a}}}}$ value.
The strong acids say HY, dissociates completely in the solution and exists as ${{H}^{+}}$ and ${{Y}^{-}}$ ions, they dissociate completely. But for weak acids like HX only partial dissociation takes place and hence in the solution it exists in dissociated and in undissociated states,${{H}^{+}},{{X}^{-}}$ and ${{H}^{+}},{{X}^{-}}$ states respectively and the two states will be in equilibrium.
- For strong acids, the strength may be represented using ${{K}_{a}}$, but for weak acids the ${{K}_{a}}$ will be too small and will be raised to power of 10 and it's very inconvenient for the representation.
For example ${{K}_{a}}$ for acetic acid is $1.76\times {{10}^{-5}}$, but ${{P}_{{{K}_{a}}}}$ value is 4.75, which is easy for representing.

So to find the questions for the answer let’s compare the ${{P}_{{{K}_{a}}}}$ values of the given compounds
${{P}_{{{K}_{a}}}}$ Values of:
A) Peroxyacetic acid-8.2
B) Acetic acid-4.76
C) Chlororacteic acid-2.75
D) Tricholoacetic acid-0.65
E) Propanoic acid.-4.87
Greater the ${{P}_{{{K}_{a}}}}$, weaker the acid.
Hence peroxyacetic acid is the weakest acid in the given options.
So, the correct answer is “Option A”.

Note: The relation between ${{K}_{a}}$ and ${{P}_{{{K}_{a}}}}$ is,
${{P}_{{{k}_{a}}}}=-\log {{K}_{a}}$
Always take the value of ${{K}_{a}}$ for the comparison of acidity of strong acids and the value of ${{P}_{{{K}_{a}}}}$ for the weak acids.