[student0808]

I have been trying to solve this for hours, with no luck.

Consider a solution of an acid-base indicator with an acid dissociation constant (Ka) of
1.42 X 10-5M, and a total analytical concentration Ctotal. For all practical purposes the
indicator is entirely in the acid form in 0.1 M HCl; that is, Ctotal = [HIn]. Likewise, in
0.1 M NaOH, the indicator is completely in its conjugate base form, and Ctotal = [In-
].
The measured molar absorptivities (ε) (L mol-1 cm-1) of the indicator species at 430 nm and 570 nm are:

At wavelength 430: HIn(in 0.1M HCl)=630, In-(in 0.1M NaOH)=20600
At wavelength 570. HIn=7120, In-=960
 

Part a) If we make up four solutions of the indicator using HIn (not in HCl or NaOH) such
that Ctotal = 2.00 x 10-5 M, 2.00 x 10-4 M, 2.00 x 10-3 M, and 2.00 x 10-2 M what are the values of [In-] and [HIn] in each of the solutions?

Part b) Calculate absorbances of each solution at each wavelength in a 1cm cell

I really don't even know where to start with this. I know that Ka= [H+][In-]/[HA], but I don't know how to proceed. I also know to use A=ebC for part b, but don't know how to apply it.

[Borek]

I know that Ka= [H+][In-]/[HA]

More precisely,

[tex]K_a = \frac {[H^+][In^-]}{[HIn]}[/tex]

Can you solve it for [itex]\frac{[In^-]}{[HIn]}[/itex]? That will give you ratio of concentrations. You also know what is the sum of concentrations of [In^-] and [HIn]...