Be sure to answer all parts. Tris [tris(hydroxymethyl)aminomethane] is a common

Question

Be sure to answer all parts.

Tris [tris(hydroxymethyl)aminomethane] is a common buffer for studying biological systems.

(Ka = 5.01 ×109 and pKa = 8.3)

(a) Calculate the pH of the Tris buffer after mixing 10.5 mL of 0.20 M HCl solution with 25.0 mL of 0.10 M Tris.



(b) This buffer was used to study an enzyme-catalyzed reaction. As a result of the reaction, 0.00018 mol of H+ was produced. What is the pH of the buffer at the end of the reaction?



(c) What would be the final pH if no buffer were present?

Explanation / Answer

a)

mmol of TRIS = MV = 25*0.1 = 2.5

mmol of acid = MV = 0.2*10.5 = 2.1

after reaction

TRIS-H+ = 2.1

TRIS left = 2.5-2.1 = 0.4

pH = pKa + log(TRIS-H+ /TRIS)

pH = 8.3+log(0.4/2.1) =

pH = 7.579

b)

addition of = 0.00018 mol of H+ = 0.18 mmol

TRIS-H+ = 2.1 +0.18 = 2.28

TRIS left = 0.4-0.18 = 0.22

pH = 8.3 + log(0.22/2.28) = 7.2844

c)

final pH if no buffer... V = 25 mL

[H+] = mol/V = (0.18)/25) = 0.0072

pH = -log(0.0072) = 2.142

pH will drop drastically

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