Question Video: Identifying the Graph Showing Change in pH When Mixing an Acid with a Base Chemistry

In an experiment, a solution of 1 m H₂SO₄ was placed into a beaker. NaOH solution was added to the beaker at regular intervals, and the pH was continually measured. Which of the graphs shows how the pH would change during this experiment?

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Video Transcript

In an experiment, a solution of one molar H2SO4 was placed into a beaker. NaOH solution was added to the beaker at regular intervals, and the pH was continually measured. Which of the graphs shows how the pH would change during this experiment?

We have been given four options labeled from (A) to (D). H2SO4 is sulfuric acid, and one molar is the concentration of the sulfuric acid. NaOH is sodium hydroxide, so we want to figure out how the pH would change when sodium hydroxide is added to sulfuric acid. First of all, we need to know what the initial pH is before the sodium hydroxide has been added. The pH is a number that indicates the acidity or basicity of a solution. Most species will have a pH between zero and 14. Species with a pH between zero and seven are acidic, whereas species with a pH between seven and 14 are basic. Species with a pH of seven are neither acidic nor basic; they are neutral. An example of a neutral species is pure water.

We’ve been told in the question that we’re starting with sulfuric acid. Since an acid has a pH between zero and seven, it is likely that the initial pH before any sodium hydroxide has been added will be small, thus can be found in the lower half of the 𝑦-axis. Of the graphs given in options (A) to (D), the initial pH for two of the options is very high. This is the case for options (B) and (C). So, it’s very unlikely that either option (B) or (C) will be the answer to this question. Options (A) and (D), however, have a low starting pH. So, the answer to this question could be (A) or (D). We’ve established that as we’re starting with an acid, the initial pH is going to be quite low. But what happens when we start adding sodium hydroxide? Sodium hydroxide is a base. An indicator of this is that it contains hydroxide ions.

The reaction of an acid and base produces a salt and water. It is called a neutralization reaction. Addition of a small amount of base will neutralize a small amount of acid. So, the solution will become slightly less acidic, and the pH would increase but only slightly. As we continue to increase the volume of sodium hydroxide added, the pH will continue to slowly increase as more and more of the acid is neutralized. Eventually, there’ll be a sudden large change in pH. This is the point at which an acid and base have completely neutralized one another. It is called the equivalence point.

If we continue to add sodium hydroxide, the base, beyond this point, then the pH will continue to rise. There will be a large jump, but then the pH would level out. If we join these data points together, we can see that of options (A) and (D), the plot that we have drawn more closely resembles the plot in option (D). So, the answer to the question “Which of the graphs shows how the pH would change during this experiment?” is (D).

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