comparing pH and pKa
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comparing pH and pKa
For Achieve #9 and #10 (week 2), we were asked to compare pH and pKa to find out which species is predominant in the reaction. Could someone explain why this comparison can be made? (when pH = pKa, when pH > pKa, and when pH < pKa)
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Re: comparing pH and pKa
When pKa is large, Ka is small, which means the equilibrium favors the backward reaction. Comparing to the environmental pH, if the environmental pH value is small, then there are more H+ / less OH-, which can shift the equilibrium, causing onside has more molecules than the other side before reaching the equilibrium.
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Re: comparing pH and pKa
Hi Akane,
The dissociation of a weak acid is Ka= [H+][A-]/[HA]. To understand what is happening mathematically, I think understanding the Henderson-Hasselbalch equation would be helpful. We can derive the equation through the dissociation of a weak acid equation to get pH=pKa + log[A-]/[HA].
This equation relates pH and pKa.
So if pH is less than pKa we know that the later part of the equation (log[A-]/[HA]) must be LESS than 0. which means that [A-]/[HA] is LESS THAN 1 (due to logarithm properties). And thus [HA]>[A-].
If pH is greater than pKa we know that the later part of the equation (log[A-]/[HA]) must be GREATER than 0. which means that [A-]/[HA] is GREATER THAN 1 (. And thus [A-]>[HA].
And pH=pKa when [A-] (the conjugate base) is EQUAL TO [HA] (the weak acid).
I hope this was a little helpful!
The dissociation of a weak acid is Ka= [H+][A-]/[HA]. To understand what is happening mathematically, I think understanding the Henderson-Hasselbalch equation would be helpful. We can derive the equation through the dissociation of a weak acid equation to get pH=pKa + log[A-]/[HA].
This equation relates pH and pKa.
So if pH is less than pKa we know that the later part of the equation (log[A-]/[HA]) must be LESS than 0. which means that [A-]/[HA] is LESS THAN 1 (due to logarithm properties). And thus [HA]>[A-].
If pH is greater than pKa we know that the later part of the equation (log[A-]/[HA]) must be GREATER than 0. which means that [A-]/[HA] is GREATER THAN 1 (. And thus [A-]>[HA].
And pH=pKa when [A-] (the conjugate base) is EQUAL TO [HA] (the weak acid).
I hope this was a little helpful!
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