The following %T data were obtained for a phenol red solution at 560 nm. Calcula
ID: 495252 • Letter: T
Question
The following %T data were obtained for a phenol red solution at 560 nm. Calculate the absorbances that correspond to the %T values. When two drops of concentrated HCl were added to the pH 7.13 solution, the indicator was converted to pure HIn form. Us measured absorbance, A_HIn, was 0.0003. When two drops of 50% NaOH were added to the pH 8.56 solution, the result was pure In form. The measured absorbance of this solution, A_In^-, was 0.712 Calculate the pK_a value for each solution and determine the average pK_a. Calculate K_a from the average pK_a.Explanation / Answer
Calulate Absorbance (A) using the formula, A=2 – log(%T)
Calcualte log [(A-Aln-)/(AHln-A)] using the A values and Aln- = 0.712 & AHln = 0.0003
Calculate pKa using the formula, pKa=pH - log [(A-Aln-)/(AHln-A)]
Calculate Ka from pKa using the formula, Ka=10-pKa (as pKa=-log [Ka])
pH %T A log [(A-Aln-)/(AHln-A)] pKa Ka 7.13 71.8 0.144 0.597 6.533 2.931 X 10-7 7.39 58.4 0.234 0.311 7.079 8.337 X 10-8 7.79 39.6 0.402 -0.113 7.923 1.194 X 10-8 8.21 28.2 0.549 -0.527 8.737 1.832 X 10-9 8.56 23.6 0.627 -0.868 9.428 3.733 X 10-10