Consider how to prepare a buffer solution with pH = 7.13 (using one of the weak

Question

Consider how to prepare a buffer solution with pH = 7.13 (using one of the weak acid/conjugate base systems shown here) by combining 1.00 L of a 0.435-M solution of weak acid with 0.288 M sodium hydroxide.

HNO2

NO2-

4.5 x 10-4

3.35

HClO

ClO-

3.5 x 10-8

7.46

HCN

CN-

4.0 x 10-10

9.40

How many L of the sodium hydroxide solution would have to be added to the acid solution of your choice? _____L

Weak Acid Conjugate Base Ka pKa

HNO2

NO2-

4.5 x 10-4

3.35

HClO

ClO-

3.5 x 10-8

7.46

HCN

CN-

4.0 x 10-10

9.40

Explanation / Answer

we will chose acid as HClO as its pKa is closer to required pH
pKa = 7.46

use:
pH = pKa + log {[ClO-]/[HClO]}
7.13 = 7.46 + log {[ClO-]/[HClO]}
log {[ClO-]/[HClO]} = -0.33
[ClO-]/[HClO] = 0.4677

let volume of NaOH added be V L

them mol of NaOH added = 0.288*V mol
mol of HClO initially = 0.435 mol

they will react,
mol of ClO- formed = 0.288*V mol
mol of HClO remaining = (0.435 - 0.288*V) mol

[ClO-]/[HClO] = 0.4677
(0.288*V) / (0.435 - 0.288*V) = 0.4677
0.288*V = 0.2034 - 0.1347*V
V = 0.481 L

Answer: 0.481 L

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