PLEASE SHOW ALL WORKINGS. I HAVE ALREADY SOLVED A AND JUST NEED HELP WITH B. THA

Question

PLEASE SHOW ALL WORKINGS. I HAVE ALREADY SOLVED A AND JUST NEED HELP WITH B. THANKS IN ADVANCE

An organic compound is abbreviated Tris and is basic. The conjugate acid (TrisH+) has a pKa of 8.10.   

TrisH+ + H20 yields Tris + H30+.

Part A:   A buffer solution is prepared by mixing 12.5 mL of 0.100 mol/L HCl with 25.0 mL of 0.100 mol/L tris. What is the pH of the buffer? ANSWER TO A=8.10

Part B:   The buffer was used to study an enzyme-catalyzed reaction in which 1*10^-4 mol of H3O+ was produced. What is the pH of the solution after the acid reacted with the buffer?

Explanation / Answer

Part A:

Tris + HCl ---> TrisH+ + Cl-

Moles of HCl added = 12.5 * 0.1 = 1.25 mmol

Moles of Tris added = 25 * 0.1 = 2.5 mmol

Moles of TrisH+ formed = 1.25 mmol

Moles of Tris left = 2.5 - 1.25 = 1.25 mmol

Total volume = 12.5 + 25 = 37.5 mL

TrisH+ ---> H+ + Tris

pKa = pH - log(Tris / TrisH+)

8.1 = pH - log(1.25 / 1.25)

pH = 8.1

PART B:

Moles of H+ added from acid = 10-4 moles or 0.1 mmol

Moles of TrisH+ formed = 1.25 + 0.1 mmol = 1.35 mmol

Moles of Tris left = 1.25 - 0.1 = 1.15 mmol

pKa = pH - log(Tris / TrisH+)

8.1 = pH - log(1.15 / 1.35)

pH = 8.1 - 0.07 = 8.03

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