Which of the following is the strongest electrolyte based on the information provided?

I concur with Justaguide. Among the given electrolytes HCl is the strongest.

Electrolyte is a type of conductor which conducts electricity either in the fused state or in solution. For example, NaCl can conduct electricity in the molten state as well as in solution. Solid NaCl is, however, a bad conductor, as the ions are arranged in fixed positions and hence are immobile. Ions are the current carriers in an electrolyte.

In the question, you are asked to compare the conductance of solutions to determine the relative strengths of the electrolytes. The conductance of a solution depends upon the number of ions in unit volume of the solution. The number of ions depends upon various factors like nature of the electrolyte, nature of the solvent, degree of ionization, ionic mobility etc.Degree of ionization of an electrolyte is the fraction of the total number of molecules of it in the ionized state. It depends upon the nature of the electrolyte. For a weak electrolyte, like ethanoic acid, the degree of ionization is small in concentrated solutions. Hence its conductance will be low at high concentration. On dilution, the degree of ionization increases. As a result more ions are produced and the conductance increases.

In the case of a strong electrolyte, like NaCl, which is composed of only Na+ and Cl- ions, the degree of ionization is unity. Even for such electrolytes, the conductance increases on dilution. This is because, according to the theory of strong electrolytes, in a concentrated solution of a strong electrolyte, the ions are close to each other and as a result the force of attraction between oppositely charged ions will be strong. Due to that the velocity of each kind of ion decreases. On dilution, the ions already present, move far apart, thereby decreasing the force of attraction between them. As a result the velocity of the ions increases.

The equivalent conductance of an electrolyte is the conductance of a solution containing one equivalent ( one gram equivalent) of the electrolyte.  Its unit is S m2 (Siemens metre2). Equivalent conductance of any electrolyte increases on dilution and attains a limiting value, called equivalent conductance at infinite dilution. On further dilution, the value does not change.

From the above discussion it will be clear that conductance (or equivalent conductance) of an electrolyte is a measure of its strength. The relative strengths of different electrolytes can be obtained from the equivalent conductances of their solutions in the same solvent at the same concentration. Here equimolar aqueous solutions are used at the same temperature. The equivalent conductances of NaOH, HF, HCl and KOH are 238, 96, 412 and 228 Sm2 respectively. Hence HCl is the strongest electrolyte among them.

The option ‘a’ is the correct one.

H+ is the fastest cation and OH- is the fastest anion. Velocity of H+ is almost twice that of OH-.  Na+, K+, Cl- and F- have almost similar ionic velocities. Their velocities are almost half that of OH-. Cl- is faster than F-.

Hence HF has a lower equivalent conductance than HCl.