Question

Prepare a buffer by acid–base reactions.

Consider how to prepare a buffer solution with pH =
**9.31** (using one of the weak acid/conjugate base
systems shown here) by combining **1.00** L of a
**0.316**-M solution of weak acid with
**0.351** M **potassium hydroxide**.

Weak Acid | Conjugate Base | K_{a} |
pK_{a} |
---|---|---|---|

HNO |
NO |
4.5 x 10 |
3.35 |

HClO |
ClO |
3.5 x 10 |
7.46 |

HCN |
CN |
4.0 x 10 |
9.40 |

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

L

Answer #1

moles of weak acid = 0.316 x 1 = 0.316

moles of KOH = 0.351 x V = 0.351 V

**the best combination HCN + CN-** . because its
pKa value nearer to pH value

pKa = 9.40

pH = 9.31

HCN + KOH --------------------------> KCN + H2O

0.316 0.351V 0 0 -------------------> initial

0.316-0.351V 0 0.351V 0.351V ----------------> after reaction

pH = pKa + log [KCN/HCN]

9.31 = 9.40 + log (0.351 V / 0.316-0.351V)

0.813 =0.351 V / 0.316-0.351V

V = 0.404 L

**volume of KOH needed = 0.404 L**

Prepare a buffer by acid–base reactions.
Close Problem
Consider how to prepare a buffer solution with pH =
9.31 (using one of the weak acid/conjugate base
systems shown here) by combining 1.00 L of a
0.316-M solution of weak acid with
0.351 M potassium hydroxide.
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
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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
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Conjugate Base
Ka
pKa
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