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Chapter 15: Problem 98

Why does an indicator need to be a weak acid or a weak base?

Short Answer

Expert verified
An indicator needs to be a weak acid or a weak base because it establishes an equilibrium in solution. Changes in pH during an acid-base titration shift this equilibrium, causing the indicator to change color and signal the endpoint of the titration.

Step by step solution

01

Understanding Indicators

An indicator is a substance that changes its color in response to a change in pH. Indicators are used in acid-base titrations to signal the completion of the reaction, known as the endpoint of the titration.
02

Properties of Weak Acids and Bases

Weak acids and bases only partially ionize in water, which means they establish an equilibrium between the ionized and non-ionized forms. The ratio between these forms depends on the pH of the solution.
03

Indicator as Weak Acid

If the indicator is a weak acid, it will donate a proton (H+) and change color when the pH of the solution increases. The molecule form (HIn) and ionized form (In-) of the indicator will have different colors.
04

Indicator as Weak Base

If the indicator is a weak base, it will accept a proton (H+) and change color when the pH of the solution decreases. The base form (In) and the conjugate acid form (HIn+) of the indicator will have different colors.
05

Conclusion

An indicator needs to be a weak acid or a weak base because it needs to establish an equilibrium in solution. The shift in this equilibrium, caused by changes in pH during the titration, allows the indicator to change color and signal the endpoint of the titration.

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Most popular questions from this chapter

If 18.5 \(\mathrm{mL}\) of a 0.0350 \(\mathrm{M} \mathrm{H}_{2} \mathrm{SO}_{4}\) solution neutralizes 12.5 \(\mathrm{mL}\) of aqueous LiOH, what mass of LiOH was used to make 1.00 \(\mathrm{L}\) of the LiOH solution?

Describe two methods of measuring pH, and explain the advantages and disadvantages of each method.

The pH of a solution is \(3.0 .\) What is \(\left[\mathrm{H}_{3} \mathrm{O}^{+}\right] ?\)

What is the concentration of hydroxide ions in pure water?

Why is a buret, rather than a graduated cylinder, used in titrations?
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