when I use henderson-hasselbach

Ka of HOP4^2- = 4.8 x10^-13
pKa= -lg(4.8 x10^-13)=12.3
substituting I get
7.44-12.3 = lg [base/acid]
e^-4.86=[base/acid}= 7.75 x10^-3
according to my textbook this answer is incorrect and should be 0.59
Please Help!

IF you are trying to work the problem you posted earlier, the 7.44 pH of a soution of H2PO4^- and HPO4^=, the numbers appear to me that you are using k3. You should be using k2.

In fact, if you look at my original post you will note that I specifically wrote pK2.
pH = pK2 + log (b/a)

And if you do it that way, I found
b/a = 1.7. The problem asked for a/b so the reciprocal of that is 0.59.

It seems like there might be a misinterpretation in your calculation. Let me explain the Henderson-Hasselbalch equation and how to properly use it.

The Henderson-Hasselbalch equation is a mathematical relationship that relates the pH of a solution to the pKa and the ratio of the concentration of a weak acid and its conjugate base. The equation is given as:

pH = pKa + log([conjugate base]/[weak acid])

Here's how you can correctly apply the Henderson-Hasselbalch equation:

1. Determine the pKa value: In your case, the pKa value is provided as 12.3.

2. Calculate the ratio [conjugate base]/[weak acid]: The ratio depends on the concentrations of the weak acid (H2PO4-) and its conjugate base (HPO4^2-). However, you have not provided the specific concentrations in your question. Please check if you have the concentrations of both species (H2PO4- and HPO4^2-) available. Without the concentrations, it won't be possible to accurately calculate the ratio or the pH.

3. Substitute the pKa value and the ratio into the Henderson-Hasselbalch equation: Once you have the concentrations, you can substitute them into the equation.

It's important to note that the Henderson-Hasselbalch equation can only provide an accurate pH if the concentrations are known. Without the concentrations, it's not possible to calculate the answer. Please make sure you have all the necessary information before proceeding with the calculation.

If you have any further questions or require additional information, please feel free to ask.