Question

The ionization of p-nitrophenol is shown below (pKa = 7.0)

a.(4 points)At pH 7, what are the relative concentrations of ionized and un-ionized p-nitrophenol?

b.(4 points)If enough concentrated hydrochloric acid is added to a solution of p-nitrophenol to lower the pH from 7 to 5, what will happen to the relative concentrations of the ionized and un-ionized forms?

c.(4 points) Ionized p-nitrophenol has a yellow color, while the un-ionized form is colorless. The yellow color can be measured using a spectrophotometer at 400nm. In order to determine the total amount of p-nitrophenol in a solution, would you perform the spectrophotometer reading at an acidic or basic pH? Clearly explain why?

d.(8 points) A solution of p-nitrophenol at pH 8.45 was found to have an A₄₀₀ of 0.370 . What is the total concentration (in µM) of p-nitrophenol (ionized plus un-ionized) in the solution? The molar extinction coefficient of p-nitrophenol is 17,500 M^-1cm^-1 and the pKa is 7.