Question

You prepared four standard solutions by serial dilutions that contained decreasing concentrations of bromocresol green, but constant HCl concentration. You also prepared four additional standard solutions by serial dilutions that contained decreasing concentrations of bromocresol green, but constant NaOH concentration. You then measured the absorbances of these solutions at their respective lamba max wavelengths (AX1 & A2). These absorbances were plotted versus Hin and In molar concentrations respecti vely to produce linear calibration curves with linear equations shown below. AX1 = 0.26conchin + 0.023 An2 = 0.38concin + 0.031 You also mixed 35 mL of your unknown with 25 mL of bromocresol green solution and diluted to the mark in a 100 mL volumetric flask. You measured the absorbance of this solution at both wavelengths (Axlunk & AX2unk) Axlunk = 0.398 Anzunk = 0.521 Given that the Ka of bromocresol green is 1.6 x 10-6, calculate the pH of your original unknown solution. Report your pH with 2 significant figures.

Answer 1

Concentration of indicator (C) = (0.398-0.023)/0.26 = 1.44 M

pH = 1/2 (pKa - LogC)

Here, pKa = -Log(Ka) = -Log(1.6*10^-6) = 5.8

i.e. pH = 1/2 (5.8 - Log1.44)

i.e. pH = 1/2 (5.8 - 0.16)

Therefore, pH = 2.82