Acid–base indicators Knowledge-Based Practice

Questions

1. 1.

   Why is only a minimal quantity of an acid–base indicator, typically two to three drops, added to the analyte solution during a titration?

   • A) High indicator concentrations permanently inhibit the neutralization reaction by forming stable coordination complexes with the analyte ions.
   • B) Excess indicator molecules rapidly decompose in aqueous solution and release gas bubbles that interfere with reading the burette.
   • C) Large amounts of indicator shift the theoretical equivalence point of the titration by altering the ionic strength of the solvent.
   • D) Indicators are themselves weak acids or bases that consume titrant, so large amounts would introduce significant titration error.
2. 2.

   Which indicator is most suitable for detecting the equivalence point in the titration of a 0.10 M weak monoprotic acid with a 0.10 M strong base?

   • A) Methyl orange (transition range pH 3.1–4.4)
   • B) Methyl red (transition range pH 4.4–6.2)
   • C) Phenolphthalein (transition range pH 8.2–10.0)
   • D) Bromothymol blue (transition range pH 6.0–7.6)
3. 3.

   An acid–base indicator $\text{HIn}$ is red in its molecular form and yellow in its ionized form $\text{In}^-$, with a $\text{p}K_{\text{a}}$ of 8.2. Which color will this indicator display when added to a solution with a pH of 10.0, and what is the relationship between the concentrations of its two forms?

   • A) Red, because $[\text{HIn}] > [\text{In}^-]$
   • B) Yellow, because $[\text{In}^-] > [\text{HIn}]$
   • C) Red, because $[\text{In}^-] > [\text{HIn}]$
   • D) Orange, because $[\text{HIn}] = [\text{In}^-]$
4. 4.

   A synthetic acid–base indicator, $\text{HIn}$, has an acid dissociation constant of $K_{\text{In}} = 1.0 \times 10^{-5}$. Over which approximate pH range does this indicator undergo its visible color transition?

   • A) pH 1.0 to 3.0
   • B) pH 2.5 to 4.5
   • C) pH 4.0 to 6.0
   • D) pH 6.0 to 8.0
5. 5.

   Which of the following statements correctly describes the chemical nature and operating principle of an acid–base indicator?

   • A) They are strong acids that completely dissociate to shift the equivalence point of a titration.
   • B) They are neutral salts that change color irreversibly upon reacting with hydronium ions.
   • C) They are weak acids or bases whose protonated and deprotonated forms have distinctly different colors.
   • D) They are redox-active compounds whose color changes depend exclusively on electron transfer regardless of solution pH.

Answers and step-by-step solutions appear after you finish the test.