Animated Solution for Chemistry - Ionic Equilibrium: In an acid-base titration, 0.1 M HCl solution was added to the NaOH solution of unknown strength. Which of the following correctly shows the change of pH of the titration mixture in this experiment?

Imagine you are standing in a pristine chemistry laboratory. In front of you is a classic setup: a glass beaker resting on a white tile, and above it, a tall, graduated burette.

Inside the beaker is a solution of sodium hydroxide ($\text{NaOH}$). We don't know its exact concentration, but we know its nature—it is a strong base.

Inside the burette is our titrant: a $0.1\text{ M}$ solution of hydrochloric acid ($\text{HCl}$), a notoriously strong acid.

Our goal in this experiment is to slowly add the acid into the base and monitor the pH of the mixture. The question asks us to identify the correct graphical representation of this pH change. To solve this, we don't need complex calculations; we need to understand the physical story of the ions in that beaker.

Before we even touch the stopcock of the burette, let's think about the environment inside the beaker.

Sodium hydroxide is a strong electrolyte. When dissolved in water, it dissociates completely into sodium ions (${\text{Na}}^{+}$) and hydroxide ions (${\text{OH}}^{-}$).

Because there is a high concentration of hydroxide ions, the solution is highly alkaline. On the pH scale, which ranges from $0$ to $14$, a strong base will sit very high up, typically around a pH of $13$ or $14$.

This single observation is a powerful tool. If we look at the four given graphs, we can immediately eliminate any graph that starts at a low pH. Graph (B) and Graph (D) start near a pH of $1$, which would imply we are starting with an acid. Since we are starting with a base, our correct graph must begin at the top of the y-axis. This leaves us with Graph (A) and Graph (C).

Now, we gently open the stopcock and let the hydrochloric acid drip into the beaker.

What happens at the molecular level? The hydrochloric acid introduces hydrogen ions (${\text{H}}^{+}$) into the solution. These hydrogen ions are highly reactive and immediately seek out the hydroxide ions.

When they collide, they undergo a neutralization reaction:

Every drop of acid consumes some of the base, turning it into neutral water and dissolved sodium chloride ($\text{NaCl}$). As the concentration of hydroxide ions decreases, the pH begins to drop.

However, initially, this drop is very gradual. Why? Because there is such a massive excess of hydroxide ions in the beaker that a few drops of acid barely make a dent in the overall concentration. The curve remains relatively flat at the top.

As we continue to add acid, we eventually reach a critical moment: the equivalence point.

At this exact moment, we have added just enough acid to perfectly neutralize all the base originally present in the beaker. There is no excess acid, and no excess base. The only things left in the solution are water and sodium chloride.

Because sodium chloride is a salt formed from a strong acid and a strong base, it does not undergo hydrolysis. It is perfectly neutral. Therefore, at the equivalence point, the pH of the solution is exactly $7$.

But the most fascinating part of a strong acid-strong base titration is how it reaches this point. Just before the equivalence point, there is a tiny amount of unreacted base left. Just one more drop of acid wipes out this remaining base and introduces an excess of hydrogen ions.

Because pH is a logarithmic scale ($pH=-\log [{\text{H}}^{+}]$), going from a tiny amount of base to a tiny amount of acid causes the hydrogen ion concentration to change by several orders of magnitude instantly.

This results in a massive, sharp, vertical drop on the titration curve. The pH plummets from around $10$ down to $4$ in the span of a single drop!

After the equivalence point, we are simply adding more and more strong acid to the beaker.

The solution becomes highly acidic, and the pH levels off at a very low value, typically around $1$ or $2$.

If we look back at our remaining options, Graph (A) perfectly illustrates this entire story. It starts high, remains relatively flat, undergoes a sharp vertical drop passing exactly through pH $7$, and then levels off at a low pH.

Graph (C), on the other hand, shows a bizarre curve that drops and then rises again, which defies the laws of chemistry for a simple continuous addition of acid.

Therefore, Graph (A) is the undisputed correct representation of this titration mixture.

Final Conclusion: The correct option is (b), which corresponds to Graph (A). Always remember that the shape of the titration curve is a direct signature of the strengths of the acid and base involved!