[foxshreiber]

We are trying to find the pH at the first equivalence point of the titration of 40 mL of .500M H3PO4 with 1.0M NaOH.

At the first equivalence point, I found the pH to be about 7.8, using an ICE chart to find the concentration of H2PO4 at the first equivalence point, and then using the Kb value for the equation
H2PO4- + H2O <> H3PO4 + OH-
since H3PO4 is a weak acid and NaOH is a strong base. I found the concentration of OH- and then took the pOH and found pH from there.


Can someone tell me where I went wrong? Many people are telling me the answer is around 1.2 and I don't understand how that can be.

[AWK]

The first equivalence point for H₃PO₄ is at a pH of about 5.

[foxshreiber]

Yes, I understand now that my answer is incorrect, but I am confused on the work that is done to get that answer. I am able to get the concentration of H2PO4- at the first equivalence point, but I don't understand how to get the pH from there.

Also, why is the pH less than 7 if you are titrating a weak acid (H3PO4) with a strong base (NaOH)

Thanks for your help.

[AWK]

https://lh3.googleusercontent.com/proxy/P3_tqzoC2QUdU887yvvrcqINgQJDcRjdt9RuxbHg7Kb1_9abHs547Rt-emh9FOm1l1GardHUmwYZpPeum84gp4Zws1Ip0fqvIgwPr4qhDdxnGFj2xv7qer-TGJqaGgvJnA

[foxshreiber]

https://lh3.googleusercontent.com/proxy/P3_tqzoC2QUdU887yvvrcqINgQJDcRjdt9RuxbHg7Kb1_9abHs547Rt-emh9FOm1l1GardHUmwYZpPeum84gp4Zws1Ip0fqvIgwPr4qhDdxnGFj2xv7qer-TGJqaGgvJnA

didn't realize it was that simple - thanks for your help