The of a certain indicator is . The color of HIn is green and that of is red. A few drops of the indicator are
added to an HCl solution, which is then titrated against an NaOH solution. Over which pH range will the indicator change
color? Select the single best answer.
4.2 to 6.2
5.2 to 7.2
6.2 to 8.2
2.2 to 4.2
3.2 to 5.2

To determine the pH range over which the indicator changes color, we need to use the value. The indicates the equilibrium constant for the dissociation of the weak acid form of the indicator. Given that , we can calculate the pH range using the Henderson-Hasselbalch equation.

For (green) and (red), the color change occurs around the pKa, which can be found using the formula . The color change will typically occur within one pH unit above and below the pKa. Therefore, the color transition range would be approximately from 4.2 to 6.2.

So the best answer is 4.2 to 6.2.

The pKa tells us a lot about how indicators behave in different solutions, and this one is no exception. It’s fascinating how these compounds visibly signal a change in acidity, and it’s not just limited to lab experiments. They play crucial roles in many real-world applications, such as monitoring soil pH for agriculture or even in your daily life, like ensuring the proper acidity in food products for better taste.