Question

Sample Buffer Calculation:

Given a pH of 5.4, prepare a buffer using one of the buffer systems below:

System A: 3.5M acetic Acid (MM=60.1) : sodium acetate, anhydrous (MM=82.0) pKa = 4.74

System B: Sodium phosphate, monobasic (NaH2PO4 x H2O) (MM=138.0) : Sodium phosphate, dibasic, Na2HPO4 (MM=142.0) pKa = 6.86

a) Choose the buffer system most effective at pH of 5.4

b) Calculate the amounts of weak acid and conjugate base you need to make 25mL of 0.25M solutions (one acid and one base solution). If you are using acetic acid, then you will have to calculate volume needed, not amount.

c) Using the Henderson-Hasselbalch equation, determine the proportions of weak acid and conjugate base required for the buffer at the assigned pH

Answer 1

A) System A is the most effective, since its component has a pKa closer to the required pH.

B) The volume of acetic acid and the mass of acetate required are calculated:

V acid = V2 * M2 / M1 = 25 mL * 0.25 M / 3.5 M = 1.8 mL

m acetate = M * V * MM = 0.25 M * 0.025 L * 82 g / mol = 0.51 g

C) The molar ratio of the components, required:

n Acetate / n Acid = 10 ^ (pH - pKa) = 10 ^ (5.4 - 4.74) = 4.5

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