Determine the pKa value from the titration curves

[Mitchtwitchita]

This is a topic that I simply know very little about. The question is asking me to determine the pKa value from the titration curves that I graphed in a recent experiment. Although I know the "buffer region," I have no idea how to determine the pKa. The weak base was 1M, 0.75M, 0.50M, and 0.25M of 30mL sodium carbonate and HCl was the strong acid added in increments of 1mL at a time, whereupon a pH was taken. Can anybody please help? I need as much help as I can get at this present time! Also, why doesn't the starting concentration of sodium carbonate affect the determination of the pKa? Is it because it's a base?

 

[symbolipoint]

The middle of the flat region (might be slope small but not quite zero) of the buffer region gives you the pKa. As for the pH of the untitrated base solution not giving you a pKa value; a weak base has a pKb, not a pKa. The initial pH of the sodium carbonate solution tells you nothing about the results of titration. You must perform the titration and take data points of pH versus titrant volume.

What have you studied so far about acid-base equilibria?

 

[Mitchtwitchita]

Thank you so much for your reply! I've barely studied anything regarding acid/base equilibrium. This was an experiment in biology concerning buffers and I can't say that my chemistry was quite up to snuff. Thanks again Symbolipoint.