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Chapter 17: Problem 34

How does titration of a strong, monoprotic acid with a strong base differ from titration of a weak, monoprotic acid with a strong base with respect to the following: (a) quantity of base required to reach the equivalence point, (b) \(\mathrm{pH}\) at the beginning of the titration, \((\mathbf{c}) \mathrm{pH}\) at the equivalence point, \((\mathbf{d}) \mathrm{pH}\) after addition of a slight excess of base, (e) choice of indicator for determining the equivalence point?

Short Answer

Expert verified
Both strong and weak monoprotic acids require the same amount of base to reach the equivalence point, while the pH differs at various stages of titration. Strong acids have a lower initial pH, a pH of 7 at the equivalence point, and a greater pH after excess base addition, compared to weak acids. Strong acids can use phenolphthalein or bromothymol blue as indicators, whereas weak acids should use phenolphthalein due to its pH range above 7.

Step by step solution

01

a) Quantity of base required to reach the equivalence point

In both cases, the quantity of base required to reach the equivalence point is the same. This is because the stoichiometry of the acid-base reaction remains the same. In other words, one mole of acid will react with one mole of base for both strong and weak monoprotic acids.
02

b) pH at the beginning of the titration

The initial pH for a strong acid will be lower than that for a weak acid, as strong acids fully ionize in water, producing a higher concentration of \(\mathrm{H}^+\) ions, which results in a lower pH. On the other hand, weak acids only partially ionize, producing fewer \(\mathrm{H}^+\) ions, which results in a higher initial pH value.
03

c) pH at the equivalence point

For the titration of a strong acid with a strong base, the pH at the equivalence point is typically 7, as the salt that forms will not hydrolyze. For the titration of a weak acid with a strong base, the pH at the equivalence point will be greater than 7, as the conjugate base of the weak acid will hydrolyze in water, producing a basic solution.
04

d) pH after the addition of a slight excess of base

For both cases, the pH will be greater than 7 after the addition of a slight excess of base, as more base was added than required to reach the equivalence point. However, the pH will typically be higher for the titration of a weak acid with a strong base, as the conjugate base of the weak acid will continue to hydrolyze in water, producing an even more basic solution.
05

e) Choice of indicator for determining the equivalence point

For the titration of a strong acid with a strong base, an indicator with a pH range around 7, like phenolphthalein or bromothymol blue, is suitable. For the titration of a weak acid with a strong base, an indicator with a pH range above 7, like phenolphthalein, should be chosen, as it will change color around the equivalence point, which will have a pH greater than 7.

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Most popular questions from this chapter

What is the \(\mathrm{pH}\) of a solution made by mixing \(0.30 \mathrm{~mol}\) \(\mathrm{NaOH}, 0.25 \mathrm{~mol} \mathrm{Na}_{2} \mathrm{HPO}_{4},\) and \(0.20 \mathrm{~mol} \mathrm{H}_{3} \mathrm{PO}_{4}\) with water and diluting to \(1.00 \mathrm{~L} ?\)

Derive an equation similar to the Henderson-Hasselbalch equation relating the \(\mathrm{pOH}\) of a buffer to the \(\mathrm{p} K_{b}\) of its base component.

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