Electrolyte is a solution and a medium that consists of free ions which help in the conduction of electricity. The solute in an electrolyte will break up from its molecular form to form free ions. A strong electrolyte consists of a solute that dissociates into free ions in large quantity while a weak electrolyte does not release much of the free ions. Some of the examples of strong electrolyte are sodium nitrate, sodium chloride and sodium sulphate and one example for weak a electrolytes is ammonia solution.
Weak electrolytes are solutions that have the substances dissolved in them in the form of molecules rather than ions. Ammonia in water is an example for weak electrolyte. It exists as molecule in water and to some extent get dissociated as ion. Since the weak electrolytes have fewer ions in the solution, it acts as weak conductor of electricity. The weak electrolyte consists of ions and molecules in equilibrium with each other. They exist as molecules as well as dissociate back into ions. The reactants (molecular form) and the products (ionic form) will be in equilibrium. Hence enough free ions are lacking to conduct electricity. In the case of hydrogen chloride, the hydrogen and chlorine get dissociated and form cation and anion. These ions do not get converted back into HCl again. As the ions exist as such, the solution of HCl will have ample ions to conduct electricity and hence acts as a strong electrolyte.
The equation given below shows the dissociation of ammonia into ions and vice versa.
NH3 (aq.) + H2O = NH4+ (aq.) + OH- (aq.)
This equation works out in both the directions. The cation and anion that are formed to conduct electricity will not stay back as such. They get immediately converted into ammonia and water. This is the reason for ammonia to act as weak electrolyte.