Hello,

Question

I have data here:

tube # Contents Initial color (after 2 drops of universal indicator) Color after adding HCl Color after adding NaOH (after the HCl had been poured)
1 CO32-/HCO3- Dark purple Purple Purple
2 CO32-/HCO3- Dark purple -- --
3 HCH3CO2/C H3CO2- red Light red Purple
4 HCH3CO2/C H3CO2- Red -- --
5 NAHCO3 Blue-green Light green Light blue
6 HCH3CO2 Red brighter red Orange
7 Distilled H2O Light green Bright red purple

A better version is at...
tinypic. com/ view. php?pic=20tqyv8&s=5. (Tubes 1 and 2 each had actual pHs of 9.5, while tubes 3 and 4 had 3.5, which is something not mentioned on the table.) How do I find the expected initial pH of the buffered solutions (tubes 1 and 3) with this data and the molarities (M) of NaCH3CO2, NaHCO3, and Ns2CO3, which are .1002, .09999, and .09992 respectively?

I was thinking about ICE tables or the Henderson-Hassalbalch equation, but I don't have any Ka values. Should I look up pH values online that correspond to colors like dark purple, blue, red, etc. and just use those as the expected initial values? That would give me a very large percent error because of possible large ranges...

Answer 1

First, I tried the tiny pics and didn't get anything.
Second, from the chart which is difficult to understand due to spacing problems, I think you can use the HH equation, substitute the concns you have listed in your narrative as well as the pH you have and that will allow you to calculate the pKa value. You don't have a question (as to what you are trying to do) but with a pKa value I'm sure you can work something out.

Answer 2

Hi Dr. Bob! I actually cleared it all up, but thank you!