Biochemistry pka and pH relationship

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SUMMARY

The relationship between pKa and pH in weak acid solutions is defined by the Henderson-Hasselbalch equation: pH = pKa + log([A-]/[HA]). When a solution contains more HA than A-, the pH is less than the pKa, contradicting the initial assertion that pH > pKa. For example, if pH is 4 and pKa is 5, the ratio of A- to HA is 0.1:1, indicating a predominance of HA. Therefore, the correct conclusion is that pH < pKa when HA is greater than A-.

PREREQUISITES
  • Understanding of weak acids and their dissociation
  • Familiarity with the Henderson-Hasselbalch equation
  • Basic knowledge of pH and pKa concepts
  • Ability to interpret acid-base equilibria
NEXT STEPS
  • Study the Henderson-Hasselbalch equation in detail
  • Explore examples of weak acid dissociation and their pH calculations
  • Learn about buffer solutions and their pH stability
  • Investigate the implications of pKa in biochemical reactions
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Students in biochemistry, chemistry educators, and anyone interested in understanding acid-base equilibria and their applications in biological systems.

sharp531
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Homework Statement


If a solution of a weak acid contains more HA than A-, then


Homework Equations



-

The Attempt at a Solution



the answer is pH > pKa.

Okay my question is why is this the answer. I'm trying to understand in what situation you would have the pH greater or less than the pKa and what that would mean in terms of the amount of A- and HA. The Henderson-Hasselbach equation is


pH = pka + log [A-]\[HA]

So let's say the pH is 6 and pka is 5. Then your ratio of A-:HA will be 10:1. That's pretty easy. This example would contain more A- than HA. So here, pH > pKa correct? Why is that then the answer to the original question was pH > pKa when you have more HA than A-??
Like if your pH was 4 and your pKa was 5, then the ratio would be 0.1:1 for A-:HA and mean that there is more HA? Then that would mean pKa>pH. Maybe I'm misinterpreting this!
 
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sharp531 said:
If a solution of a weak acid contains more HA than A-, then pH > pKa.

Don't waste you time trying to twist the reality. This statement is false.

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