Question

The highest electrical conductivity of the following aqueous solution is of:
(a) 0.1 M acetic acid
(b) 0.1 M chloroacetic acid
(c) 0.1 M fluoroacetic acid
(d) 0.1 M difluoroacetic acid

Answer 1

Hint: The electrical conductivity of an electrolytic solution mainly depends on its concentration, temperature and nature of the electrolyte (whether the electrolyte is weak or strong).

Complete answer:
The electrolytic conductance of an electrolyte is its ability to allow the passage of electrical current through it. The electrolytes conduct electricity due to the presence of ions in their aqueous solution or in molten state.
There are certain factors which affect the electrolytic conductance of an electrolyte:
Concentration of the ions: Higher the concentration of the ions, greater will be conductivity of the electrolytic solution.
Nature of the electrolyte: Strong electrolytes show more electrolytic conductance than the weak electrolytes. This is because strong electrolytes completely dissociate in their aqueous solutions whereas weak electrolytes partially dissociate in their aqueous solutions as is evidenced by their small value of the dissociation constant.
Temperature; Higher the temperature, higher will be the electrolytic conductance. High temperature increases the mobility of the ions and increases the dissociation of weak electrolytes.

In the above question, the concentration of each acid given is the same, therefore ideally each acid should give the same amount of the ions in their aqueous solutions, but all these acids are not equally strong. We can deduce which acid will have the highest conductivity by referring to their $ { pK }_{ a }$ values and the stability of the conjugate base in the aqueous medium. Acetic acid is a weak acid with a $ { pK }_{ a }$ of 4.8. The conjugate base of chloroacetic acid is more stable than the acetate ion since the chlorine atoms present in the conjugate base of chloroacetic acid stabilise it due to the inductive effect of chlorine; the $ { pK }_{ a }$ of chloroacetic acid is 2.87.

The conjugate base of fluoroacetic acid is more stable than that of chloroacetic acid since fluorine is more electronegative than chlorine and shows stronger inductive effect. The $ { pK }_{ a }$ of fluoroacetic acid is 2.6. The conjugate base of difluoroacetic acid is more stable than that of fluoroacetic acid since the former contains two fluorine atoms which will exert more inductive effect than in the latter and help in the stabilisation of the conjugate base. The ${ pK }_{ a }$ of difluoroacetic acid is 0.025.

So, the correct answer is “Option (D) 0.1 M difluoroacetic acid”.

Note: If the concentration of the acids given in the question were different, then we would have to calculate the degree of dissociation of the particular acids in order to know the number of ions in the solution. Also the nature of the ions (their charge to radius ratio) will also become a crucial factor while considering the electrolytic conductivity of the acids.