Final answer:
Option c is the correct answer. The pH remains stable when a small amount of acid or base is added to a NH3 and NH4Cl solution because it acts as a buffer. A buffer contains a weak base and its conjugate acid, which can neutralize added strong acids or bases, resulting in only slight changes in pH.
Step-by-step explanation:
The pH does not change significantly when a small amount of an acid or a base is added to a solution containing equal amounts of the base NH3 and a salt of its conjugate acid NH4Cl because the system is a buffer. A buffer consists of a mixture of a weak acid or a weak base and the salt of its conjugate. The buffering action is a result of the system being able to convert the added strong acid or base into its weaker, conjugate forms. For instance, when a strong acid is added to a buffer that consists of NH3 and NH4Cl, NH3 neutralizes some of the added acid to form additional NH4+, which only slightly changes the pH of the solution. Similarly, when a strong base is added, the NH4+ ion in NH4Cl reacts with the OH- ions to form NH3 and water, which again results in a minimal change of the overall pH in the solution. This demonstrates the reason why option (c) The system is a buffer, with the base and its conjugate acid in equal amounts, is the correct explanation.