1. The problem statement, all variables and given/known data Did a lab where I had to determine the Ka value of an indicator using spectrophotometry. This is a question I need to answer in my lab report: Derive the equation pH = pKa + log([A-Aa]/[Ab-A]) from the equation pH = pKa + log( [In-]/[HIn]) using Beer's law. A: measured absorbance at λb at specified pH Aa: absorbance of acidic form at λb Ab: absorbance of basic form at λb λb: wavelength of maximum absorbance for basic form 2. Relevant equations 3. The attempt at a solution A1 = ε1ab[HIn] + ε1bb[In-] A2 = ε2ab[HIn] + ε2bb[In-] Aa = εab[HIN] Ab = εbb[IN-] Ab/Aa = εbb[IN-] / εab[HIN] Rearrange: [In-] / [HIn] = εaAb / εbAa A = Aa+Ab so [In-] / [HIn] = εa(A-Aa) / εb(A-Ab) I'm stuck. I have been working on this for hours and cannot figure out how to get to the answer. Mainly, I just don't know how to get the molar absorbtivities to help me because they are all different. I'm so confused. Can someone steer me in the right direction? I also tried solving using the first two equations: A1 = ε1ab[HIn] + ε1bb[In-] A2 = ε2ab[HIn] + ε2bb[In-] which led me to [In-] / [HIn] = (A1ε2a-A2ε1a ) / (A2ε1b - A1ε2b) which doesn't help me either because I can't see how to get rid of the molar absorbtivities. HELP Thank you.