4. Recall the experiment \"The equilibrium constant of bromothymol blue (HBB): H

Question

4. Recall the experiment "The equilibrium constant of bromothymol blue (HBB): HBB(aq) BB (aq)+H (aq) Knowing that: For strongly basic solution: [HBB(ag) 0, [8B (aq) [HBBlaa)lo Recall that Beer's law relates absorbance and concentration: A Ebc, where E is the extinction coefficient, b is the pathlength of the light passing through the sample. (10 pts) Determine the value of Keq for the ionization of bromothymol blue from the given data. Extinction coefficient for BB at 635 nm is 9.10x10 Mcm a) HBB] A 635nm [BB] pH [H Keq of BB 0.750 8.00x10M 6.87 (10 pts) A student prepares the solution A-2 according to the following procedure: Transfer 10.0 mL of 8.00x10 M A-1 solution to a dry 50.0 mL volumetric flask. Then fill the volumetric flask to the fill mark with solution A. What is the concentration of A-2 solution? b)

Explanation / Answer

Concentration [BB]0 = 8.00 x 10 -5 M

A = 0.750

Since A = ?bc, [BB-] = A/?b, expecting b = 1, extinction coefficient ? = 9.10 x 103 M-1 cm-1

Concentration of [BB-] = 0.750 / 9.10 x 103 = 8.24e-5 M

pH = 6.87, pH = -log[H3O+]

[H3O]+ = 10-6.87 = 1.34896288e-7 M

[HBB(aq)] = [BB-(aq)]

Keq = [BB-(aq)] x [H3O]+ / [HBB(aq)]

Keq = 8.24e-5 x 1.348e-7 / 8.20e-5

Keq = 1.355e-7

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