[contour_pl]

Hello, I was hoping someone could give me some ideas on how to approach this question, as I am very lost.

A mixed aqueous solution of HCl and phosphorous acid (H3PO3; Ka1 = 3.00×10-2; Ka2 = 1.66×10-7) is titrated with 0.1000-M NaOH.
The first end point (cresol purple indicator) is reached after 35.22 mL of NaOH has been added.
The second end point (thymol blue indicator) is reached after a total of 54.26 mL of NaOH has been added.

What quantity of HCl (in mol) was present in the initial mixture?

I wrote the expressions for Ka1 and Ka2, in hopes to determine the amount of H+ dissociated from the phosphorous acid. However, there seem to be too many unknowns, in addition to the fact that, even if I determine how much H+ is present at hte first end point, I cannot determine how much of it came from HCl.

Any guidelines?
Thanks.

[Borek]

What will be neutralized at the first end point?

What will be neutralized at the second end point?

Hint: take a look at titration curves generated with BATE.

[contour_pl]

So at the first endpoint, the H+ from the HCl and the first H+ from phosphoric acid is neutralized? Thus, at the second endpoint, the second H+ is neutralized?

If this is the case, then at the second endpoint, I can determine the amount of H+ from the second donation of protons from phosphoric acid?

[Borek]

2*yes.