2020 P1 Q15 - Determine Suitable Indicator for Equivalence Point

Let's take a look at 2020 A Levels H2 Chemistry Paper 1 Question 15.

2020 p1 q15 question

We need to determine which indicators can be used to show the first and second equivalence points for the titration of triprotic acid H3PO4 with NaOH.

The working range of the indicator is its pH range of colour changes.

This info is given in the question.

The equivalence point is the midpoint of the region of rapid pH change for the titration curve.

From the titration curve in the question, we can determine that:

1. First equivalence point is at 50.0cm3, range of rapid pH change is around pH 3 to 6
2. Second equivalence point is at 100.0cm3, range of rapid pH change is around pH 8 to 11

An indicator is suitable when its working range lies within the range of rapid pH change for the titration curve.

We can highlight the working ranges for all the indicators in the titration curve to see which ones are suitable.

2020 p1 q15 compare range of rapid pH change and working range

Naphthyl red is suitable to show the first equivalence point, since its working range of pH 3.7 to 5.0 is within range of rapid pH change of pH 3 to 6 for first equivalence.

Thymol blue is suitable to show the second equivalence point, since its second working range of pH 8.0 to 9.6 is within range of rapid pH change of pH 8 to 11 for second equivalence.

We can now look at the options and determine that option B is the answer to this question.

2020 p1 q15 answer

Topic: Ionic Equilibria, Physical Chemistry, A Level Chemistry, Singapore

Back to list of questions for 2020 A Level H2 Chemistry Paper 1.

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