Chemistry 12 (HL)
Unit 5 / IB Topic 18.5
Acids and Bases – Part 8
Indicators – Practice Problems - ANSWERS
1.
Which statement about indicators is always correct?
a)
b)
c)
d)
2.
The mid-point of an indicator’s colour change is at pH = 7.
The pH range is greater for indicators with higher pH values.
The colour red indicates an acid solution.
the pKa value of an indicator is within its pH range.
Bromocresol green has a pH range of 3.8 to 5.4 and changes colour from yellow to blue as the pH
increases.
a)
Suggest a value for the pKa for this indicator. (Pretend that you do NOT have Table 16
for reference! Reason this one out and be able to explain it to someone else.)
pKa ~4.6
b)
c)
3.
The pKa is near the midpoint of the transition range.
Complete this table for the different types of acid-base titrations and state whether or not
bromocresol green would be a suitable choice as an indicator for each type.
titration type
pH at equivalence
point
Is bromocresol green a
suitable indicator?
strong acid – strong base
7
no
weak acid – strong base
8-10
no
strong acid – weak base
4-5
yes
weak acid – weak base
7
no
What is the colour of the indicator at pH 3.5?
yellow
The titration curves for a strong acid – strong base
titration and a weak acid – strong base titration are
shown at the right.
a)
Comment on the suitability of the
phenolphthalein as an indicator for
each type of titration.
It can be used for both titrations because its pH
transition range falls on the steep vertical rise of
the titration curve. This means it will change
colour quickly over a very very small addition of
base.
b)
Suggest an alternative suitable indicator for each titration and explain your selection.
with HCl:
methyl red à transition range = 4.8 to 6.0 à falls on the steep vertical section
bromothymol blue à transition range = 6.0 – 7.6 à falls on the steep vertical
section
p. 1
Chemistry 12 (HL)
Unit 5 / IB Topic 18.5
phenol red à transition range = 6.6 – 8.0 à falls on the steep vertical section
with HAc (i.e. CH3COOH) à no other suitable indicator
à transition ranges are not on the steep vertical section so the
colour will change gradually as more base is added
4.
Given this titration curve:
a)
What type of titration is shown?
weak base + strong acid
b)
Would phenolphthalein be a suitable
indicator for this titration? Explain.
No – its pH transition range is 8.2 -10.0.
The colour will change gradually as a large
volume of base is added. There will NOT be a
sharp clear endpoint.
c)
Suggest two indicators that might be suitable for this titration.
Explain your choices.
methyl red à transition range = 4.8 – 6.0
bromocresol green (3.8 – 5.4)
bromophenol blue (3.0 – 4.6)
methyl orange (3.2 – 4.4)
Identify a limitation when using the suggested indicator.
An intermediate colour may form that would be difficult to see the same way in every titration.
p. 2