BTB is most appropriate for which titration?

Answer: a strong acid - strong base titration where the equivalence pH is near 7 Indicator should change color near the equivalence pH; BTB (range 6.0-7.6) matches strong-strong titrations.

Why is the equivalence pH greater than 7?

Answer: The solution at equivalence is sodium acetate, whose acetate ion hydrolyzes to give OH- At equivalence the solution contains the salt of a weak acid (NaCH3COO). The acetate ion is a weak base (CH3COO- + H2O <- CH3COOH + OH-), giving pH 7.

Which indicator is most suitable for finding the equivalence point?

Answer: Phenolphthalein (range 8.2-10.0) Equivalence pH = 8.72 (basic because acetate is conjugate base of weak acid). Phenolphthalein's range covers this; the other indicators change color far before equivalence.

AP Chemistry · Topic 8.5

Acid-Base Titrations Practice

Q: Which indicator is most suitable for finding the equivalence point?

Answer: Phenolphthalein (range 8.2-10.0) Equivalence pH = 8.72 (basic because acetate is conjugate base of weak acid). Phenolphthalein's range covers this; the other indicators change color far before equivalence.

Part of Acids and Bases.(SAP-9.E)

Practice questions

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Sample questions

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Sample 1difficulty 1/5
$A student investigates the pKa of bromothymol blue (BTB), a weak-acid indicator that is yellow as HIn and blue as In-. Equal concentrations of BTB are placed in a series of buffered solutions and absorbance is recorded at 615 nm (the In- absorbance peak). The midpoint of the absorbance vs pH curve - where [HIn] = [In-] and the solution looks green - occurs at pH = 7.10.$
$BTB is most appropriate for which titration?$
- A
  $a strong acid titrated with a weak base (equiv pH ~5)$
- B
  $a strong acid - strong base titration where the equivalence pH is near 7$
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- C
  $any titration regardless of equivalence pH$
- D
  $a weak acid titrated with a strong base (equiv pH ~9)$
Why
$Indicator should change color near the equivalence pH; BTB (range ~6.0-7.6) matches strong-strong titrations.$
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Sample 2difficulty 1/5
$A student titrates 25.00 mL of 0.100 M acetic acid with 0.100 M NaOH while monitoring pH. Selected pH values: at 0.00 mL added, pH = 2.87; at 12.50 mL (half-equivalence), pH = 4.74; at 25.00 mL (equivalence), pH = 8.72.$
$Why is the equivalence pH greater than 7?$
- A
  $Excess NaOH is present at equivalence$
- B
  $The solution at equivalence is sodium acetate, whose acetate ion hydrolyzes to give OH-$
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- C
  $All titration equivalence points are basic$
- D
  $Acetic acid is a strong acid$
Why
$At equivalence the solution contains the salt of a weak acid (NaCH3COO). The acetate ion is a weak base (CH3COO- + H2O <-> CH3COOH + OH-), giving pH > 7.$
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Sample 3difficulty 1/5
$A student titrates 25.00 mL of 0.100 M acetic acid with 0.100 M NaOH while monitoring pH. Selected pH values: at 0.00 mL added, pH = 2.87; at 12.50 mL (half-equivalence), pH = 4.74; at 25.00 mL (equivalence), pH = 8.72.$
$Which indicator is most suitable for finding the equivalence point?$
- A
  $Bromocresol green (range 3.8-5.4)$
- B
  $Methyl red (range 4.4-6.2)$
- C
  $Methyl orange (range 3.1-4.4)$
- D
  $Phenolphthalein (range 8.2-10.0)$
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Why
$Equivalence pH = 8.72 (basic because acetate is conjugate base of weak acid). Phenolphthalein's range covers this; the other indicators change color far before equivalence.$
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Sample 4difficulty 2/5
$A 25.0 mL sample of 0.100 M HCl is titrated with 0.100 M NaOH. pH is measured every few mL.$
$What is the pH at the equivalence point?$
- A
  $7.00$
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- B
  $5.00$
- C
  $1.00$
- D
  $9.00$
Why
$Strong acid + strong base titration: equivalence pH = 7.00. The salt formed (NaCl) is neutral.$
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Sample 5difficulty 2/5
$A 25.0 mL sample of 0.10 M HCl is titrated with 0.10 M NaOH.$
$What is the pH at the equivalence point?$
- A
  $greater than 7$
- B
  $less than 7$
- C
  $Cannot be determined$
- D
  $7$
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Why
$Strong acid + strong base produces a neutral salt (NaCl) and water. The equivalence-point pH is 7.$
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