Question

Consider how to prepare a buffer solution with pH =
**9.52** (using one of the weak acid/conjugate base
systems shown here) by combining **1.00** L of a
**0.417**-M solution of weak acid with
**0.284** 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. |

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

?L

Answer #1

*Given that*

*pH = 9.52*

*Concentration of KOH = 0.284 M*

*moles of weak acid = 0.417 x 1*

*= 0.417*

*moles of KOH = 0.284 x V*

*= 0.284 V*

*As its pKa value nearer to pH value , the best combination
is HCN + CN- .*

*pKa = 9*

*Let us consider a reaction*

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

*0.417 0.284V 0 0*

*0.417-0.284V 0 0.284V 0.284V*

*We know,*

*pH = pKa + log [KCN/HCN]*

*9.52 = 9 + log (0.284V) / (0.417-0.284V)*

*log (0.284V) / (0.417-0.284V) = 3.31*

**V = 1.13 L**

*volume of KOH needed = 0.404 L*

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Weak Acid
Conjugate Base
Ka
pKa
HNO2
NO2-
4.5 x 10-4
3.35
HClO
ClO-
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7.46
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CN-
4.0 x 10-10
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Consider how to prepare a buffer solution with pH =
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systems shown here) by combining 1.00 L of a
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Conjugate Base
Ka
pKa
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3.5 x 10-8
7.46
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CN-
4.0 x 10-10
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