Required Practical 1 - Volumetric Solution + Titration

Question 1

Volumetric Solution

Answer 1

• Weigh known mass of solid using weighing bottle and record value
• Wash all into beaker using distilled water
• Dissolve in beaker by stirring with glass rod
• Wash out into a 250cm3 volumetric flask
• Add dropwise with teat pipette to marking/meniscus
• Fasten and invert flask several times

Question 2

Titration

Answer 2

• Pipette a known volume of solution of an unknown concentration into a conical flask
• Add a few drops of phenolphthalein indicator
• Add volumetric solution to burette using a funnel
• Record start volume to 2 dp
• Titrate solution from burette until first permanent colour change
• Record rough titre to 2 dp
• Repeat until three results are recorded within 0.1cm3 to calculate a mean titre

Question 3

Suggest a reason for having a rough titre

Answer 3

Allows a rough estimate to made for the titre so repeats can be titrated dropwise near that value to obtain an accurate result

Question 4

Give a reason why rough titres are not included in calculating mean titre

Answer 4

Not within 0.1cm3 of other titres

Question 5

Give hazards and safety precautions for acid-base titrations

Answer 5

• corrosive so wear eye protection/gloves

- irritant vapour so handle in fume cupboard

Question 6

Suggest how you could reduce percentage uncertainty of titre and justify your choice

Answer 6

• More concentrated standard solution in conical flask

- Larger titre reading

Question 7

Improvements for preparation of volumetric solution

Answer 7

• Wash out weighing bottle/beaker/stirring rod/funnel using distilled water (into volumetric flask)
• Use a teat pipette to make up mark on volumetric flask
• Invert flask

Question 8

Purpose of titration

Answer 8

• determine concentration of a basic solution

- detect equivalence point for a acid-base reaction

Question 9

Suggest why it is not important to fill burette to zero mark

Answer 9

Volumes are determined from difference between initial and final readings

Question 10

Suggest significance of touching off hanging drop after closing burette

Answer 10

Liquid on tip of burette is part of volume delivered by burette

Question 11

Suggest why you should not leave the funnel in the burette

Answer 11

• small drops of liquid might fall into the burette

- mean titre is lower than actual titre

Question 12

Suggest why burette and pipette are rinsed with reagents they hold before titration

Answer 12

• ensure only appropriate reagent present
• prevent contamination or dilution of solution by water or other chemicals
• removed any air bubbles trapped in valve

Question 13

Suggest why two or three drops of indicator are added to conical flask

Answer 13

• too little means colour is not visible

- too much could impact pH thus cannot determine true end point of reaction

Question 14

Suggest why it is important to take burette reading at eye level or below meniscus

Answer 14

Avoid parallax error

Question 15

Suggest why a conical flask is used in a titration rather than a beaker

Answer 15

Easier to swirl without losing liquid

Question 16

Suggest why a burette is appropriate to use in an acid base titration

Answer 16

• can control amount of liquid released
• allows for extremely accurate addition of liquid
• precise readings can be taken

Question 17

Purpose of volumetric pipette

Answer 17

Measuring and transferring an exact volume

Question 18

Explain what is meant by concordant results

Answer 18

• within 0.1 od each other

- reproducible, consistent results

Question 19

Suggest how to obtain an accurate average titre

Answer 19

• do further titrations (at least three)

- must be concordant

Question 20

Suggest how you can accurately measure volume in burette

Answer 20

• take reading at eye level or below meniscus

- hold while tile behind burette so it is easier to read

Question 21

Explain why using a burette increases the precision of the titration

Answer 21

• burette has high precision (scale measure in 0.1cm^3)

- precision is limited by scale of instrument used

Question 22

Suggest why distilled water is used to wash conical flask near end point of titration

Answer 22

• returns reagent on sides of flask to reaction mixture

- ensure reagent is full reacted to give accurate end point

Question 23

What effect will adding distilled water to conical flask near end point of titration have on titre

Answer 23

• no effect
• number of moles of solution does not change (only diluted)
• colour change determined by moles of solution

Question 24

Suggest why concordant results are used to calculate mean titre

Answer 24

• within 0.1cm^3 of each other
• more reliable mean titre calculated
• reduced human error in titration

Question 25

Suggest why conical flask should be rinsed with distilled water before titration

Answer 25

- holds certain number of moles of standard solution | - rinsing removes any residues that could add to the number of moles of solution

Question 26

Suggest effect of bubble in burette on titre reading

Answer 26

- titre reading will be higher than actual titre | - when bubble bursts volume decreases but no reagent is added

Question 27

How to calculate percentage error of equipment

Answer 27

Sum of individual percentage errors of equipment used

Question 28

Describe steps taken during the titration procedure that help to ensure that accurate results are obtained

Answer 28

- conical flask rinsed with distilled water - burette and pipette rinsed with reagents they hold - funnel is removed once solution is transferred to burette - only a few drops of indicator added to conical flask - conical flask is swirled - ensure burette has no air bubbles - readings for burette are taken at eye level or below meniscus - white tile held behind burette when readings taken - distilled water used to wash sides of conical flask near end point - solution is added dropwise near end point of titration

Question 29

Suggest why it is important to fill the space below the burette tap with solution before beginning a titration

Answer 29

- space will fill during titration - volumes added will be too large - titre larger than true value

Question 30

Suggest why it is important to carry our repeat titrations

Answer 30

- repeats improve reliability | - single titration could be anomalous