4.11 Titrations
5 exam questions worth 7 marks. Write your answer, open the mark scheme, then give yourself the marks you earned. Your answers are saved on this device.
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Why is universal indicator not used in titrations?
Choose one answer.
Correct Not quite: the answer is Its colour changes gradually
Mark scheme
B
Model answer
B: universal indicator changes colour gradually, so the end point is not sharp.
Name the piece of apparatus used to measure exactly 25.0 cm3 of alkali into the flask.
Mark scheme
- (volumetric) pipette (1)
Model answer
A volumetric pipette (with a pipette filler).
A student's titres were 23.40, 22.95, 23.05 and 23.00 cm3. The first was a rough titration.
Calculate the mean titre using concordant results.
Mark scheme
- concordant: 22.95, 23.05, 23.00 (1)
- mean = 23.00 cm3 (1)
Model answer
Ignore the rough titre; 22.95, 23.05 and 23.00 are within 0.10 cm3, so the mean is 69.00 ÷ 3 = 23.00 cm3.
Give two things a student should do to make sure the end point is found accurately.
Mark scheme
any two from:
- add acid drop by drop near the end point (1)
- swirl the flask (1)
- use a white tile (1)
- read the burette at eye level / bottom of meniscus (1)
Model answer
Add the acid drop by drop near the end point while swirling the flask, and use a white tile to see the colour change clearly.
Explain why the rough titration is not included when calculating the mean titre.
Mark scheme
- it is less accurate / the end point is usually overshot (1)
Model answer
The rough titration is done quickly to find the approximate end point, so it usually overshoots and is less accurate.
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