AP Chemistry · Topic 8.7

pH and pKa Practice

Part of Acids and Bases.(SAP-10.A)

Practice questions

2

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Sample questions

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Sample 1difficulty 2/5
$A student investigates the pKa of bromothymol blue (BTB), a weak-acid indicator that is yellow as HIn and blue as In-. Equal concentrations of BTB are placed in a series of buffered solutions and absorbance is recorded at 615 nm (the In- absorbance peak). The midpoint of the absorbance vs pH curve - where [HIn] = [In-] and the solution looks green - occurs at pH = 7.10.$
$What is the pKa of bromothymol blue from this measurement?$
- A
  $0.50$
- B
  $7.10$
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- C
  $14.00$
- D
  $3.55$
Why
$For an indicator HIn <-> H+ + In-, when [HIn] = [In-], pH = pKa. Midpoint pH 7.10 = pKa.$
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Sample 2difficulty 2/5
$A smaller pKa means$
- A
  $Weaker acid$
- B
  $Stronger acid$
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- C
  $Weaker conjugate base$
- D
  $Larger pKb$
Why
$pKa = -log K_a; smaller pKa = larger K_a = stronger acid.$
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