Question

The molar absorptivities at 430 and 570 nm of the weak acid Hin (K. = 1.42 x 105) and its conjugate base In were determined by measurements of strongly acidic and strongly basic solutions of the indicator. Under this conditions, essentially all of the indicator was in the Hin and Inform, respectively. The molar absorptivities of Hin and In at 430 nm & 570 nm were 6.30 x 102 and 7.12 x 103, and 2.06 x 10' and 9.61 x 102, respectively. Calculate absorbance data for unbuffered solutions that have total indicator concentrations ranging from 2 x 105 to 16 x 10-6 M.

Answer 1

The calculation of absorbance for a chemical indicator in acid-base titrations involves the application of the Beer-Lambert Law, using the provided molar absorptivities at different wavelengths and considering the indicator's form based on the pH conditions.

Step-by-step explanation:

The student's question involves calculating the absorbance of different concentrations of a chemical indicator used in acid-base titrations. The indicator consists of a protonated form (HIn) and a conjugate base (In-), each having specific molar absorptivities at two different wavelengths (430 nm and 570 nm). To calculate the absorbance for each concentration of the solution, one would apply the Beer-Lambert Law, which states that absorbance (A) is proportional to concentration ([C]) times the path length (l), times the molar absorptivity (ε): A = ε * l * [C].

This equation will be applied independently for each form of the indicator (HIn and In-) at the given concentrations, using the provided molar absorptivities for 430 nm and 570 nm, respecting that at strongly acidic or basic conditions, nearly all of the indicator is in the HIn or In- form, respectively. The resulting absorbance values will indicate the color intensity of the solution, which is important for understanding the indicator's pH titration characteristics and selecting appropriate indicators for different titrations.