Typical Concept Problems 1) For a weak acid, or a buffer, why is the half-equiva

Question

Typical Concept Problems 1) For a weak acid, or a buffer, why is the half-equivalence point important? 2) What is true at the equivalence point of the titration of a weak monoprotic acid? Why is the pH above 7? 3) Why should dilution of a buffer with pure water not change the pH? 4) Why would a mixture of NH3 and NHaCl constitute a buffer; why would a mixture of HC and NHs constitute a buffer; why would a mixture of Na0H and NHa not constitutea buffer? 5) Will a mixture of HCl and NaOH make a buffer? Why or why not? 6) Why is CaC0s more soluble in HCI than in pure water?

Explanation / Answer

1) For a weak acid the half equivalence point means , where half of the acid is neutralised.

HA + OH- -------------> A-

1 0 0 initial

0.5 - 0.5 at half equivalence point

This point is important as [acid] = [conjugate base] and

the pH of buffer = pKa of the weak acid

The buffer capacity of the solution is maximum at this stage.

2)At the equivalence point of titration of weak monoprotic acid

the solution does not become neutral , but the solution is basic.

It is due to te formation of a salt of weak acid and strong base, which undergoes anionic hydrolysis.

Thus the solution is basic and the pH of the solution > 7

3) The pH of buffer is calculated using Hendersen equation

pH = pKa + log [conjugate base]/[acid]

thus pKa of acid being unchanged , the pH of buffer is determined by the ratio of [conjugate base]/[acid],

On dilution the concentrations of both conjugate base and acid are affected by same factor, making their ratio same. Thus dilution does not affect the pH of buffer.

Related Questions

Navigate

• Browse (All)
• Browse T
• Subjects

---

---

Integrity-first tutoring: explanations and feedback only — we do not complete graded work. Learn more.