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Edexcel IAL Chemistry 8.22 Volumetric uncertainty and technique

Account for both burette readings, divide absolute uncertainty by the measured quantity and link each rinsing, mixing or endpoint choice to moles or concentration.

Syllabus
First assessment 2019
Course
Chemistry YCH11
Level
AS

Exam points

  • Calculate titre percentage uncertainty using two burette-reading uncertainties.
  • Explain how rinsing and dilution affect delivered moles, concentration or titre.
  • Improve endpoint accuracy with concordant runs, swirling and dropwise addition near the endpoint.

8.22—How to minimise the sources of measurement uncertainty in volumetric analysis and estimate the overall uncertainty in question 1

[Maximum number: 3]

Sodium hydroxide solution reacts with carbon dioxide in the air and should be standardised before use. Ethanedioic acid may be used for this standardisation.

Question (a)

(a)

A different standard solution of ethanedioic acid is used to determine the concentration of a sodium hydroxide solution J.

Procedure

Step 1 A burette is rinsed with deionised water.
Step 2 The burette is then rinsed with 0.0900 moldm−30.0900 \mathrm{~mol} \mathrm{dm}^{-3} ethanedioic acid and filled with this acid solution.

Step 3 A pipette is used to transfer 25.0 cm325.0 \mathrm{~cm}^{3} portions of solution J to conical flasks.
Step 4 The portions are titrated with the ethanedioic acid solution using phenolphthalein indicator.

[ 3 ]

Question (i)

(i)

Explain why the burette is rinsed with ethanedioic acid solution in Step 2.

[ 1 ]

Question (ii)

(ii)

The student completely emptied the pipette for each transfer in Step 3.

Explain the effect on the titre of completely emptying the pipette rather than leaving a small amount of solution in the tip.

[ 2 ]
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