Question

The increase in the molar conductivity of HCL with dilution is due to:
(A) increase in the self-ionization of water
(B) hydrolysis of HCL
(C) a decrease in the self-ionization of water
(D) a decrease in the interionic forces

Answer 1

Hint: The conductance property of a solution containing one mole of the electrolyte is known as molar conductivity. It is the property of an electrolyte solution used to determine the efficiency of a given electrolyte in a solution to conduct electricity.

Complete step by step solution:
As we know, the conductivity is the inverse of resistivity. So, to find the conductivity of a given solution first we have to find its resistivity.
To find out any unknown resistance the most accurate method is using a setup similar to Wheatstone bridge. However, generally, we face two problems while measuring the resistance of an ionic solution. They are as follows:
(1)- When the current (DC) passes through the solution its composition might get changed which can cause a change in its resistance and other properties.
(2)-We cannot connect a solution to the bridge as it is made up of a metallic wire or any other solid conductor.
But these problems can easily be solved by alternate methods. For the first problem, we can use an alternating current source and for the second one, we can use a specially designed vessel called the conductivity cell.
A conductivity cell consists of two platinum electrodes coated with platinum black (metallic platinum deposited on electrodes electrochemically). The solution is confined between these electrodes which have a column length l and cross-section area A. Therefore, its resistance can be found by the equation:
\[R{\text{ }} = {\text{ }}\rho {\text{ }}\dfrac{l}{A} = {\text{ }}\dfrac{l}{{kA}}\]
Let’s have a look at the above formula. Since the length between two electrodes distance between them is fixed for a given cell, the unit l/A is a constant called cell constant and is denoted by G*.
$G * = \dfrac{1}{A} = R \times k$
Now, we have to find the resistance by using the Wheatstone bridge setup which consists of two resistances R3 and R4, a variable resistance R1 and an unknown resistance R2 in the conductivity cell. P is a suitable detector and when no current passes through it we can say that the bridge is in a balanced state. Under these conditions:
Unknown resistance \[{R_2}{\text{ }} = {\text{ }}{R_1}{\text{ }} \times {\text{ }}\dfrac{{{R_4}}}{{{R_3}}}\]
Now we can easily determine its conductivity as: \[k = \dfrac{{G * }}{R}\]
Now, as we know that the molar conductivity is denoted by the formula:
${\Lambda _m} = \dfrac{k}{c}$
where ${\Lambda _m}$ (Greek, lambda) is known as molar conductivity in S $c{m^2}mo{l^{ - 1}}$
K is the conductivity of the cell in S$c{m^{ - 1}}$, and
C is the concentration of the given solution in mol ${L^{ - 1}}$
Now, as we know that HCL is a strong electrolyte so by increasing the concentration of water it will increase the dissociation of HCL into ${H^ + }$ and $C{l^ - }$. The interionic forces between ${H^ + }$ and $C{l^ - }$in HCL become weak and it will dissociate.

Therefore, the correct answer is (D)- Decrease in interionic forces.

Note: You must be sure about the type of electrolyte i.e. either it is a weak electrolyte or a strong one. The reason for the change in conductivity in a weak electrolyte is not the same as a strong electrolyte. Weak electrolytes have lower molar conductivities and lower degree of dissociation at higher concentrations which increases steeply at lower concentrations.