Posted by Cristian on Wednesday, February 27, 2013 at 9:48pm.
Use the Henderson Hasselbalch equation twice.
First set of conditions:
pH=9.0
pka=9.6
Solve for the ratio
pH=pka+log[A-/HA]
9.0=9.6+log[A-/HA]
10^(9.0-9.6)=[A-/HA]
0.25=[A-/HA]
Meaning 25% of the solution is protanated, or 0.025 moles is A- and 0.075 moles HA
Second set of conditions
pH=10.0
pka=9.6
Solve for the ratio
pH=pka+log[A-/HA]
10=9.6+log[A-/HA]
10^(10.0-9.6)=[A-/HA]
25.0=[A-/HA]
Meaning 25% of the solution is deprotanated, or 0.025 moles is HA and 0.075 moles A-
I need to go from 0.025 moles of A- to 0.075 moles of A-, and I need to go from 0.075 moles of HA to 0.025 moles of HA. This means that I need 0.05 moles of KOH.
5M KOH=0.05 moles of KOH/L
Solve for volume,
0.05 moles of KOH/5M=L of KOH
Typos---
Change 0.025 to 0.25
change 0.075 to 0.75
change 0.05 to 0.5
Everything is correct.
Never mind, I was correct the first time.
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