Question

58 5. Calculation of molar mass: Use class-average NaOH molarity and your part B data. (Be sure this calculation is consistent with your choice reported in #1 and 2 above.) 6. Complete the following table, having consulted a table of diprotic acids; taking into account molar mass and pKa's, identify your unknown: I Molar mass pKa1 pKa2 From titration data Data from reference | 6-1 Unknown: Maleic acid t.33 7. "It is possible to show" the first equivalence point pH of a diprotic acid is equal to the average of the two pKa values. Calculate the average of the pKa values for your unknown: .16 Use your graph to determine the pH at the first equivalence point: How much of a difference is there? 8. You produced two buffer regions in the diprotic acid titration. What two pH ranges would your system be best suited for as a buffer? (Where is pH most stable? Refer to your graph.) PH 0 2,and PH 0 5 -6 9. If you had not been told that your unknown was a triprotic acid, and it actually was, how would your results for pKal, pKa2, and your molecular weight calculation be different?

work check and help with the last question. pka2 is more accurate than pka1.

Answer 1

I have checked your work. Well done! Keep going ahead.

Now, coming to the last question:

The pKa1 and pKa2 values will be obtained more than the actual or expected values for a triprotic acid. As a result, the molecular weight will be less than the actual one.

Explanation: For a triprotic acid: pKa1 < pKa2 < pKa3

But here, if you don't know that you are using triprotic acid, you will treat it like a diprotic acid and you will get pKa1 and pKa2 values for diprotic acid type, whose values are more than those for the triprotic acid.

Now, Ka is inversely related to [acid]

pKa is inversely related to Ka.

Therefore, pKa is directly related to [acid], moles of acid.

As a result of an increase in no. of moles of acid, its molecular weight decreases.