Neutralisation - Titration curves

Question 1

Draw the titration curve for a strong acid-strong base and suggest a suitable indicator.

Answer 1

• phenolphthalein suitable
• methyl orange suitable

Question 2

Draw the tritration curve for a strong acid-weak base and state a suitable indicator.

Answer 2

• Starts at pH = 1.5
• End point at pH 6
• phenolphthalein unsuitable
• methyl orange suitable

Question 3

Draw the tritration curve for a weak acid-strong base and give a suitable indicator.

Answer 3

• phenolphthalein suitable
• methyl orange unsuitable.

Question 4

What are the color pH ranges for:

1) Phenolphthalein
2) Bromothymol blue
3) Methyl orange

Answer 4

1) 8/9 - 10
2) 6 - 7.5/8
3) 3 - 4

Question 5

What makes a suitable indicator?

Answer 5

For a titration, an indicator is chosen so that the pH value of the end point is as close as possible to the pH value of the titration’s equivalence point. In practice, a suitable indicator changes colour within the pH range in the vertical section of the titration curve.

Question 6

Draw the titration curve for weak acid-weak base and explain why there is no suitable indicator.

Answer 6

As the base is added, the pH changes slowly through the equivalence point and there is no vertical section to the titration curve. An indicator would change colour gradually over a few cm^3.