Chemical Indicators Practice Questions

What is the effect on HIn indicator when NaOH is added to it?

• Equilibrium shifts to right and more of colour B is seen.
• Equilibrium shifts to left and more of colour B is seen.
• Equilibrium shifts to right and more of colour A is seen
• Equilibrium shifts to left and more of colour A is seen

Bromophenol blue changes from yellow to blue over pH of 3.0 to 4.6. Which statement is correct?

• Molecules of bromophenol blue, HIn, are blue.
• pH < 3.0, bromophenol blue contain more ion, In–, than HIn.
• pKa of bromophenol blue is between 3.0 and 4.6.
• Bromophenol blue is suitable to titrate CH₃COOH with KOH.

Which indicator would be appropriate for titrating weak base, CH₃CH₂NH₂, with nitric acid?

• Bromophenol blue (pKa = 4.1)
• Bromothymol blue (pKa = 7.3)
• Phenol red (pKa = 8.0)
• Phenolphthalein (pKa = 9.50)

Which statement about indicators is always correct?

• Mid-point of an indicator’s colour change is at pH = 7.
• pH range is greater for indicators with higher pKa values
• Colour red indicates an acidic solution
• pKa value of the indicator is within its pH range.

Acid-base indicator has pKa value of 4.0. At what pH will this indicator change colour?

• 2
• 4
• 8
• 12

Which could use phenolphthalein (pKa=9.5) and not methyl orange (pKa=3.46) as indicator?

• NaOH and HNO₃
• NH₃ and CH₃COOH
• NaOH and CH₃COOH
• NH₃ and HNO₃

Which statement about this indicator is correct?

• In a strongly acidic solution colour B would be seen.
• In neutral solution, concentration of HIn and In^– are equal.
• Suitable for titration involving weak acids and weak bases.
• None of the above

Which indicator cannot be used for titration curve with a pH jump from (2.7 - 7.8)?

• Methyl orange (pK_a 3.4)
• Methyl red (pK_a 5)
• Bromothymol blue (pK_a 7.3)
• Phenol red (pK_a 8)

Which of the following statement is FALSE?

• Indicators can be organic weak acids

• Indicators change color at its pK_a

• End point for indicator must coincide with equivalent point

• Indicator change color at a specific pH