[Jocelyn04]

I know this is meant to be quite basic, but I wonder if anyone can help me.
I did a lab assignment, which was the “Identification of an unknown weak acid by titration” where we have to find the pKa and use it with the RMM to identify the acid

I titrated 0.1M NaOH with 25cm3 of an unknown acid, plotted the graph pH Vs titre and found the equivalence point to be ~9.8cm3 and the half equivalence point to be ~pH4.7, giving me the pH = pKa. However we are told we have to use H-H equation pKa = pH + log[HA]/[A]. when I calculate it (starting pH was 3.02) I calculate the pKa to be 3.16 which is different from the graph.

When I calculated pH + log [HA]/[H+][A-] I got a pKa of ~4.7 Is this way wrong?

Where am I going wrong?  Can anyone shed some light on this for me?

[Borek]

half equivalence point to be ~pH 4.7

Show what (and how) you did. pKa is 4.7, you must be doing something wrong when you get 3.16.

[Jocelyn04]

First I calculated No. of mols NaOH at the equivalence point (9.8cm3, pH 8.31): 0.1*9.8x10^-3 = 9.8x10-4 = No. of mols Acid

Then I used this to calculated the concentration of the unknown acid at the start of the titration: 9.8x10-4/25x10^-3 = 0.0392M and the conc of the anion of the acid [A-]: 9.8x10^-4/(25+9.8)x10^-3 = 0.0282M

Then I used this in the equation: pKa = pH + log [HA]/[A] = 3.02 + log(0.0392/0.0282) = 3.02 + 0.143 = 3.163



I know I'm missing something, but I'm not sure what.

Any help is greatly appreciated

[Borek]

unknown acid at the start of the titration: 9.8x10-4/25x10^-3 = 0.0392M

Apples.

and the conc of the anion of the acid [A-]: 9.8x10^-4/(25+9.8)x10^-3 = 0.0282M

At the end of the titration. Oranges.

You are using concentrations in two different solutions in the equation that describes equilibrium in ONE solution.

To be honest, I have no idea how to "use" H-H equation in this case. You may get any point on the titration curve, calculate HA/A ratio for this point and insert it into the equation. No idea what for, as using pH = pKa in the titration midpoint is exactly the same. HA=A, HA/A = 1, log 1 = 0. Period.