[LHM]

A solution may contain NaOH, Na₂CO₃, NaHCO₃ individually or as any pair of the three analytes. A 25.00 mL aliquot of the solution is treated with a standard solution of HCl. Given the following information, what analyte(s) are present in the solution? Addition of phenolphthalein indicator (pink at pH = 10.0 and colorless at pH = 8.3) to the solution gives a pink color. The solution becomes colorless after the methyl purple indicator (green at pH = 5.4 and purple at pH = 4.8 ) to this solution gives a green color. Addition of an additional 15.30 mL of 0.1000 M HCl causes the solution to become purple in color.

A) NaOH + NaCO₃
B) NaCO₃ + NaHCO₃
C) NaHCO₃
D) NaOH

Sorry I know that it isn't great grammar and that one of the sentences in there seems like its missing a few words, but I typed it up word for word, so what it's actually supposed to be, I don't know.

As for how to do this problem, I can tell that the original solution is basic since adding phenolphthalein produces a pink color, but assuming that it meant that the solution becomes colorless after the methyl purple indicator is added to this solution, I don't understand how that happens?

[Borek]

I guess information from these page may come in handy:

http://www.titrations.info/acid-base-titration-polyprotics-and-mixtures

http://www.titrations.info/acid-base-titration-sodium-hydroxide-and-carbonate

Solution becomes colorless after some volume of HCl was added, methyl purple is added to the colorless solution and titration is continued.

[LHM]

Thanks!

After reading this, however, I do have another question: http://www.titrations.info/acid-base-titration-sodium-hydroxide-and-carbonate

On the titration curve, the first endpoint is that of NaOH. Typically, on a normal titration curve of just NaOH and HCl, the curve is really steep at the endpoint. Why is the first end point of the Warden titration barely noticeable?

[Borek]

Carbonate is buffering the solution, slowing down pH changes.