Any ideas? In conducting a titration of 75.00 mL of a certain triprotic acid wit

Question

Any ideas?

In conducting a titration of 75.00 mL of a certain triprotic acid with 0.8000M NaOH, the following information was recorded:

a. The first equivalence point was observed at pH3.96 after the addition of 12.40 mL of NaOH (total);



b. THe second equivalence point was observed after the addition of 25.00 mL of NaOH (total; the pH was measured to be 4.98.

Determine the apparent concentration of this triprotic acid and the dissociation constant for the first and second proton dissociation steps (Ka1 and Ka2), given that pKa3 is 5.19.

? Ka1= [H2PO4-] [H+] [H3PO4] Ka2=[HPO42-][H+] [H2PO4-] ka3=[PO43-][H+] [HPO42-]

Explanation / Answer

This is the reaction for first endpoint: H3PO4(s) + H2O(l) ? H3O+(aq) + H2PO4–(aq) Ka1= 7.5×10-3 This is the reaction for second endpoint: H2PO4–(aq)+ H2O(l) ? H3O+(aq) + HPO42–(aq) Ka2= 6.2×10-8 This is the third, but it cannot be seen: HPO42–(aq)+ H2O(l) ? H3O+(aq) + PO43–(aq) Ka3= 2.14×10-13

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