The absorption spectra for a weak acid, HA, recorded at pH = 2 and its conjugate

Question

The absorption spectra for a weak acid, HA, recorded at pH = 2 and its conjugate base, A–, recorded at pH = 10 are shown below.

A 100 mL aliquot of 0.197 M HA (Ka = 1 × 10–5) is titrated with 0.394 M NaOH. The progress of the titration is monitored by measuring the absorbance of the solution at 300 nm. Which of the following four spectrophotometric titration curves best represents this titration?

The absorption spectra for a weak acid, HA, recorded at pH 2 and its conjugate base, A recorded at pH 10 are shown below Absorbance of A (pH 10) Absorbance of HA (pH 2) 100.0 00.0 90.0 90.0 80.0 80.0 70.0 70,0 60,0 E 50.0 50.0 40.0 2 40.0 2 300 E 30.0 20.0 20,0 10.0 10.0 0.0 0.0 200 500 300 400 500 300 400 200 Wavelength (nm) Wavelength (nm) A 100 mL aliquot of 0.197 M HA (Ka 1 x 10 5) is titrated with 0.394 M NaoH. The progress of the titration is monitored by measuring the absorbance of the solution at 300 nm. Which of the following four spectrophotometric titration curves best represents this titration? 120 120 100 80 He 60 Scroll down to 10 20 30 40 50 60 70 80 90 100 l10120 0 20 30 40 50 60 70 80 90 100 110 120 view all four Volume of NaOH (mL) Volume of NaOH (mL) titration curves

Explanation / Answer

The PH jump near the equivalence point becomes less pronounced as the acid becomes weaker.
Since HA is weak acid, after equivalence point detection of concentration of acid would be tough
and therefore spectrophotometer might show constant values.
Option A would be the correct representation of the this titration.

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