16/3 When a solution contains a weak acid and its conjugate base or a weak base

Question

16/3 When a solution contains a weak acid and its conjugate base or a weak base and its conjugate acid, it will be a buffer solution. For a solution of a weak acid (HA) and its conjugate base (A?),

HA(aq)?H+(aq)+A?(aq)

is called a buffer and will resist a change in pH. If acid is added, the reaction shifts to consume the added H+, forming more HA. When base is added, the base will react with H+, reducing its concentration. The reaction then shifts to replace H+ through the dissociation of HA into H+ and A?. In both instances, [H+] tends to remain constant.

The pH of a buffer is calculated by using the Henderson-Hasselbalch equation:

pH=pKa+log[A?][HA]

Part A

What is the pH of a buffer prepared by adding 0.708mol of the weak acid HA to 0.305mol of NaAin 2.00 L of solution? The dissociation constant Ka of HA is 5.66

Explanation / Answer

[HA]= 0.708/ 2.00L = 0.354 M
[A-]= 0.305/ 2.00 L =0.1525 M
pKa = - log Ka =6.25
pH = 6.25 + log 0.1525/0.354= 5.88

PART B

A- + HCl = HA + Cl-
moles A- = 0.308 - 0.150=0.158
[A-]= 0.158 / 2.00 L=0.079 M
moles HA = 0.708 + 0.150 =0.858
[HA]= 0.858 / 2.00 L =0.429 M
pH = 6.25 + log 0.079 / 0.429=5.51

PART C

HA + NaOH >> A- + H2O
moles HA = 0.708 - 0.195=0.513
[HA]= 0.513 / 2.00 L =0.2565 M
moles A- = 0.305 + 0.195=0.50
[A-]= 0.5 / 2.00 L =0.25M
pH = 6.25 + log 0.25/ 0.2565=6.23

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