Use the table of K values given with this problem to choose the best weak base t

Question

Use the table of K values given with this problem to choose the best weak base to start from for making a buffer that holds the pH of the solution at 5.00. Make your selection so that you maximize the capacity of the buffer. (Found that the best K value is with NH3 Ammonia whose Kb value = 1.76 x 10-5) Assume that you want to make 300 mL of a buffer and you have already added enough of the conjugate acid salt to make the final buffer 0.60 M in this salt. How many mL of a 2.8 M solution of the weak base you selected must be added to the volumetric flask so that when filled to the mark, the pH of the buffer will equal 5.00. Use the table of K values given with this problem to choose the best weak base to start from for making a buffer that holds the pH of the solution at 5.00. Make your selection so that you maximize the capacity of the buffer. (Found that the best K value is with NH3 Ammonia whose Kb value = 1.76 x 10-5) Assume that you want to make 300 mL of a buffer and you have already added enough of the conjugate acid salt to make the final buffer 0.60 M in this salt. How many mL of a 2.8 M solution of the weak base you selected must be added to the volumetric flask so that when filled to the mark, the pH of the buffer will equal 5.00.

Explanation / Answer

In general, pH of a bufferr always may equals to pKa + / - 1. Hence Aniline with pKb value 9.4 is a suitable base to for the buffer with pH around 5.

According to Henderson's eqation -

                     pH = pKa + log [salt] / [base]

                       5.0 = 4.6 + log [salt] / [base]

Salt/base = 2.5

Let, x = volume of base should be added

Thus number of mols of base in the resulting solution = 2.8* x mmol

Number of mols of salt = 0.60 M*300 mL = 180 mmol

Thus -    2.5 = 180 mmol / 2.8 x

              x = 25 mL (volume of base should be added)

Related Questions

Navigate

• Browse (All)
• Browse U
• Subjects

---

---

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