Question

A buffer contains 0.17 mol of propionic acid (C2H5COOH) and 0.21 mol of sodium propionate (C2H5COONa) in 1.20 L.

Part A
What is the pH of this buffer?

Part B
What is the pH of the buffer after the addition of 0.02 mol of NaOH?

Part C
What is the pH of the buffer after the addition of 0.02 mol of HI?

Answer 1

Given 0.17 mol propionic acid , 0.21 mol sodium propionate in 1.20 L

We recognize the conjugate acid-base pair propionic acid (K_a = 1.3 x 10-5 and pK_a = 4.9)

Part A:

[acid] = 0.17 / 1.2 = 0.1417 M

[base] = 0.21/1.2 = 0.175 M

pH of the buffer = 4.9 + log (0.175/0.1417) = 4.9 + 0.092 = 4.992

Part B:

moles acid = 0.17 - 0.02 = 0.15

[acid] = 0.15/1.2 = 0.125 M

moles salt = 0.21 + 0.02 = 0.23

[base] = 0.23/1.2 = 0.1917 M

pH of the buffer = 4.9 + log (0.1917/0.125) = 4.9 + 0.186 = 5.086

Part C:

moles of acid = 0.17 + 0.02 = 0.19

[acid] = 0.19/1.2 = 0.1583

moles of base = 0.21 - 0.02 = 0.19

[base] = 0.19/1.2 = 0.1583

pH of the buffer = 4.9 + log (0.1583/0.1583) = 4.9

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