Video: Selecting the Graph That Matches the pH Change for Titration of Hydrochloric Acid against Potassium Hydroxide

Which graph could represent the pH of a 0.05 M solution of hydrochloric acid versus the volume of a 0.05 M solution of potassium hydroxide that is added to it? [A] Graph A [B] Graph B [C] Graph C [D] Graph D [E] Graph E

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Video Transcript

Which graph could represent the pH of a 0.05 molar solution of hydrochloric acid versus the volume of a 0.05 molar solution of potassium hydroxide that is added to it.

We’re looking to construct a graph of the pH of an acidic solution versus the volume of potassium hydroxide, which is the base that’s added to it. This means that the pH should go on the 𝑦-axis and the volume of potassium hydroxide that we’re adding should go on the 𝑥-axis. In the experiment that’s described in the question, we’re adding a base to an acid and monitoring the pH. So, this sounds like a titration experiment. So, now let’s think about how the pH of our solution will change as we slowly titrate it with more base.

Remember that the pH is defined as the negative log of the concentration of hydronium ions in the solution. A low pH means that we have a high concentration of hydronium ions in the solution, which means that the solution will be acidic. If the pH is high, that means the concentration of hydronium ions will be low, which means our solution will be basic. In a basic solution, we have a high concentration of hydroxide, or OH⁻ ions.

Let’s think about what the pH of our solution will be before we add any base. Since we’re starting off with a solution of hydrochloric acid, we’d expect the pH to be low because the solution is acidic. As we begin to add base, it will start to react with the acid and neutralize it, which will form water and a salt, in this case KCl, which is called potassium chloride. Once we start to add the base to our solution, the pH will begin to increase because the base is reacting with the acid, which means that the concentration of our acid and, therefore, the concentration of the hydronium ions in the solution are going down.

The increase in the pH will be quite slow at first because the base is being totally used up to react with the acid. The pH of our solution will begin to increase much more quickly once we get close to the equivalence point, which is where the moles of hydroxide from the base that we’ve added are equal to the moles of hydronium that were present in the original acidic solution.

Since the products of this reaction are water and a salt, which are both neutral, we’d expect the equivalence point for this reaction to occur at a pH seven. After the equivalence point, all of the acid has been used up, but we’re still adding base to the solution. This means that the pH will continue to increase since we’re increasing the concentration of hydroxide ions in the solution. Now, that we’ve walked through the whole titration experiment, we can connect the dots to finish our graph.

You should know that in this question, we were working with a monoprotic acid, which is an acid that only has one hydrogen to react with the base. If we had a diprotic acid, which is an acid that has two hydrogens, the resulting titration curve would look much different. Either way, our graph most closely resembles answer choice E, which is the correct graph to represent the pH of a hydrochloric acid solution as we add potassium hydroxide to it.

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