Biochemistry pka and pH relationship

[sharp531]

Homework Statement

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

Homework Equations

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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-A 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-A and mean that there is more HA? Then that would mean pKa>pH. Maybe I'm misinterpreting this!

 

[Borek]

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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methods

 

[Blueshift5]

The relationship between pKa and pH is important in understanding the behavior of weak acids in solution. The pKa value represents the equilibrium constant for the dissociation of a weak acid into its conjugate base and a hydrogen ion. The lower the pKa, the stronger the acid, and the higher the pKa, the weaker the acid.

In the Henderson-Hasselbach equation, the pH is equal to the pKa plus the log of the ratio of the concentrations of the conjugate base (A-) to the weak acid (HA). When the concentration of HA is higher than that of A-, the logarithm in the equation will be negative, resulting in a pH greater than the pKa. This means that the solution is more acidic, as there is a higher concentration of hydrogen ions present.

On the other hand, if the concentration of A- is higher than that of HA, the logarithm will be positive, resulting in a pH lower than the pKa. This indicates a more basic solution, as there is a lower concentration of hydrogen ions present.

In summary, when the solution contains more HA than A-, the pH will be greater than the pKa, indicating an acidic solution. When the solution contains more A- than HA, the pH will be lower than the pKa, indicating a basic solution. This relationship between pKa and pH is important in understanding the behavior of weak acids and their ability to donate or accept protons.